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Portobello mushroom burger on a board.
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5 from 1 vote

Portobello Mushroom Burger with Caramelised Onions (Vegan)

This grilled portobello mushroom burger topped with generous pile of sweet caramelised onions has fantastic meaty texture and is rich in umami flavours. Vegan hit!
Course BBQ, Dinner, Light Lunch, Vegan Burger
Cuisine American, British, International
Keyword mushroom burger, Portobello mushroom burger, vegan burger
Prep Time 40 minutes
Cook Time 15 minutes
Making caramelised onions 55 minutes
Total Time 55 minutes
Servings 4 servings
Calories 198kcal
Author Jo Allison



  • 4 portobello mushrooms cleaned and stems removed
  • 1 tablespoon rapeseed oil to grease the grill pan with


  • 2 tablespoon balsamic vinegar
  • 2 tablespoon Extra Virgin Olive Oil
  • 1 tablespoon soy sauce or tamari
  • 1 teaspoon dried oregano
  • ½ teaspoon garlic powder
  • ½ teaspoon onion powder

Caramelised Onions

  • 3 medium onions approx. 250-290g, peeled and thinly sliced
  • 1 tablespoon rapeseed oil
  • 1 tablespoon balsamic vinegar
  • ½-1 tablespoon demerara sugar

Harissa Mayo

  • 8 tablespoon vegan mayo
  • 2 teaspoon harissa paste
  • burger rolls, sliced tomatoes, lettuce etc. to serve


Portobello Mushroom Burgers

  • Start with preparing your portobello mushrooms by giving them a wipe with a damp paper/kitchen towel and removing the stems.
  • In a shallow dish that will fit all four mushroom caps mix together ingredients for the marinade.
  • Place the mushrooms in the dish (top of the caps down), brush the other side with generous amount of marinade and let them sit and soak up the flavours for 15 minutes.
  • After approximately 15 minutes flip your portobello mushroom caps top side up and brush with some more marinade. Let them sit in the dish for another 15 minutes if you have time or give them 5 minutes and get grilling.
  • Drizzle some rapeseed oil onto the grill pan and make sure it is distributed evenly (I like to brush it on).
  • Put your pan onto a medium heat and place your portobello mushroom caps bottom side down in the pan.
  • Grill for 5-7 minutes pressing down gently with a spatula from time to time to get a little bit of caramelisation going as well as to get that flat patty shape.
  • Flip your mushrooms over and repeat the process brushing some extra marinade on your mushroom caps as you go.

Caramelised Onions

  • Thinly slice some white onions and fry them in a pan with a little bit of rapeseed oil nice and slow until softened, sweet & translucent. It will take anything between 30-45 minutes.
  • Once your onions are nice and soft add tablespoon of balsamic vinegar and ½-1 tablespoon of demerara sugar. Stir and continue frying for another 5 minus or so.
  • Use as a topping for your portobello mushroom burger!

Harissa Mayo

  • Mix harissa paste with vegan mayo in a small bowl until combined. Serve with your burger.

Burger Assembly

  • Serve grilled portobello mushroom burgers in burger buns or flatbreads with your choice of additions (sliced tomatoes, lettuce, gherkins, avocado, harissa mayo etc.) and topped with caramelised onions.



  • Nutritional information is approximate and per one portobello mushroom burger with a portion of caramelised onions. Burger buns, sauces etc. haven't been included in the calculation. 
  • In the absence of grill pan, use regular frying pan or even outdoor BBQ.
  1. Don't skimp on marinade! Make sure you use every single drop for brushing over your mushrooms when marinating and using any leftovers to brush over when grilling too.
  2. Don't be afraid of your spatula and press down (gently) on your mushrooms from time to time as they cook to ensure some caramelisation which equals even more flavour and to get that flat patty shape.
  3. For criss-cross grill marks make sure to rotate your mushroom caps on the grill pan when top side down.
  4. Go to town and use two portobello mushroom caps per burger instead of one for delicious double decker experience! 


Calories: 198kcal | Carbohydrates: 16g | Protein: 3g | Fat: 14g | Saturated Fat: 2g | Sodium: 266mg | Potassium: 445mg | Fiber: 3g | Sugar: 9g | Vitamin C: 6mg | Calcium: 29mg | Iron: 1mg