Portobello Mushroom ‘Burgers’ with Crispy Onions and Miso Mayonnaise

I am all about eating meat, seafood and veggie and plant-based alternatives – I love all food equally. But while I do enjoy a lovely steak, or chop from time to time and am definitely a huge fan of seafood (have you noticed from looking at the recipes on this website?), more often than not my daily meals are vegetarian, or even fully plant-based. This vegetarian portobello mushroom burger with crispy onions and miso mayonnaise is definitely a frequently seen ‘guest’ at our dinner table and I actually enjoy it a lot more than most beef burgers out there.


If you came here expecting a recipe for actual beef burgers topped with loads of mushrooms… sorry! I am sure I will post something like that in the future, but that’s not what this recipe is all about. Any meat lover that might already be disappointed by the fact that this is a recipe for a mushroom ‘burger’ where the meat patty is replaced by a large grilled portobello mushroom (and is upset about the fact that I call it a burger at all): perhaps you should still give this recipe a chance… (or go here). These vegetarian ‘mushroom burgers’ – well, let’s be honest, they really are mushroom sandwiches, aren’t they? – do make for a great meat alternative that even non-vegetarians can really love (such as me! and you?). 


Portobello mushrooms naturally have a certain ‘meatiness’ to them, in that they are substantially thick, hearty, succulent and full of umami and when they are marinated in a flavorful marinade and grilled over high heat (even better over a fire grill), or oven-baked, they really make for a great veggie treat that (almost) makes you forget you are not consuming any meat.

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I would be lying, though, if I said that it’s just the mushroom itself that makes me so excited about this burger… sandwich. Let’s be honest, as is the case with many burgers and sandwiches, what truly completes this hand-held meal are the toppings; some stretchy mozzarella for cheesy chewiness, a big dollop of miso mayo to amp up the savory umami elements (you can find the recipe for miso mayonnaise over here), a crisp leaf of lettuce to pretend we are healthy add some freshness, and a crunchy nest of fried onions on top for… well, I think that’s obvious (Fried! Crispy! Kind of fancy-looking!).


Whether you want to call this a veggie burger, or mushroom sandwich or whether you are looking to include more vegetarian substitutes in your diet, or just enjoy eating all kinds of foods: if you love mushrooms and bold flavours, this portobello sandwich will definitely be one of your new favourite summer meals.


Tip: The portobello mushroom is best when baked on a fire grill, or griddle pan, but it is possible to simply bake them in the oven. When baking the mushrooms in the oven, they may release and/or retain a lot of fluid. While this makes for a very juicy ‘burger’, it may be a bit too much for some. Either discard some of the cooking liquids halfway through baking to allow the mushrooms to dry out a little, or remember to squeeze some of the juice out of the mushrooms before placing them on the buns.

Tip #2: This ‘burger’ can easily be turned into a fully plant-based meal by making the miso mayo by mixing about a cup (or however much sauce you want) of vegan mayonnaise with some miso paste and by replacing the mozzarella for a plant-based alternative, or even fully omitting the cheese.


Portobello Mushroom ‘Burgers’ with Crispy Onions and Miso Mayonnaise

0.0 rating
  • V
Veggie Portobello mushroom burgers with crispy fried onions and miso mayonnaise
  • Difficulty:Easy
  • Prep Time:30 mins
  • Cook Time:25 mins
  • Serves:4
  • Freezable:No

Nutrition per portion

  • For the miso mayonnaise:
  • 1 large, fresh egg yolk
  • 1 heaped teaspoon Dijon-, or coarse mustard
  • 1 tablespoon white wine vinegar
  • 250ml sunflower-, or canola oil
  • 1 tablespoon white miso paste, or more to taste
  • -
  • For the marinade:
  • 2 tablespoons soy sauce
  • 1 tablespoon white vinegar
  • 1 tablespoon sunflower oil
  • 0.5 tablespoon brown sugar
  • 1 garlic clove, crushed
  • Pinch of ground white pepper
  • -
  • For the burgers:
  • 4 large Portobello mushrooms
  • 1 ball mozzarella (around 120g)
  • 4 sesame seed hamburger buns
  • 4 lettuce leaves
  • -
  • For the crispy onions:
  • 1 medium yellow onion, very thinly sliced
  • 2 tablespoons cornstarch
  • ¼ teaspoon salt, or more to taste
  • Sunflower oil, for frying
  1. Make the miso mayonnaise according to instructions here 
  2. Make marinade: mix the soy sauce, vinegar, oil, brown sugar, crushed garlic and pinch of white pepper.
  3. Marinate the mushrooms: generously brush the marinade on both sides of the mushrooms. Place portobello mushrooms smooth side down in an oven dish and pour the remaining marinade over the stem side. Let the mushrooms marinate stem side up for 20 minutes.
  4. Grill, or bake the portobello mushrooms:  EITHER heat a griddle pan (or BBQ grill!) over medium high heat and grill mushrooms for 8 – 10 minutes on each side until cooked, OR preheat the oven to 190 degrees Celsius and bake the mushrooms for 20 minutes, flipping them once.
  5. Add the cheese: When your mushrooms are almost done cooking, tear the mozzarella ball into 4 pieces and divide over the stem-side of the mushrooms to melt your mozzarella.
  6. While your mushrooms are cooking, make the crispy onions: Toss the thinly sliced onion with cornstarch and salt to coat evenly. Fill a small saucepan, or small high-rimmed frying pan over medium-high heat with at least 2.5 - 3cm of oil. When the oil is hot, fry the crispy onions, in batches if necessary, until golden and crisp. Use tongs, or a wooden chopstick to slightly stir the onions and keep them from sticking together too much. Place fried onions on a paper towel to drain from oil. Do this step as close to the serving time as possible to keep the onions nice and crispy.
  7. Grill the buns [optional]: lightly toast the sliced burger buns for a few minutes in the oven, or a hot pan.
  8. Assemble hamburger: Spread some of the miso mayonnaise on the bottom halves of the burger buns. Place portobello mushrooms with melted mozzarella on top of the sauce, top with the lettuce, crispy onions and other half of the bun. Serve immediately.

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