Last Updated on January 1, 2026
Healthy Portobello Mushroom Burger:
OK, I fibbed just a bit. You probably wouldn’t quite have your mushroom burger like this on a 5:2 diet fast day.
The bread roll adds a whopping 150 calories, and to me, that’s a bit of a waste of calories, as it won’t help fill you up for long. But, it’s just as delicious (though not quite as photogenic) without the bread, or with just the base, like a burger open sandwich.
And, portobello mushrooms make excellent meat substitutes. There’s nothing particularly difficult in this recipe, and it’s perfectly acceptable to mix and match the toppings according to your own personal preference. I used a slice of low-calorie Leerdammer cheese, which melts beautifully and has around 50 calories per slice, but you could crumble a little blue cheese or add a slice of grilled bacon. Or fill the cap with chopped veg and then top with cheese. It’s up to you!
The mushroom itself has around 30 calories. You will be adding a little oil to the marinade and grilling it, and of course, the mixture of toppings. So the final dish, without a bun and using just mustard and low-calorie mayo, is around 200 calories. Add the bun, and you are still only hitting 350 calories. It just doesn’t feel like diet food, though.
Portobello Mushroom Burger – 5:2 Diet Plus
Ingredients
- 1 Large Portobello Mushroom
- 1/2 Thin slice Leerdamer light cheese Or similar
- 2 teaspoons Balsamic vinegar
- 2 teaspoons Olive oil
- 1/2 teaspoon Rose Harrissa
- 1/2 Medium Red Onion finely sliced
- 1 teaspoon Olive oil
- 1 Thin slice Tomato
- 1 teaspoon Low-calorie Mayo
- 1 Handful Salad Greens
- 1 Burger Bun
Instructions
- Mix the balsamic, 2 teaspoons of oil and rose harissa. Use to baste the mushroom and put to one side for 20 minutes or so
- Gently fry the red onion in a teaspoon of olive oil till caramelised. Put to one side and keep warm
- Grill the mushroom on a griddle or barbecue, top side up for 2-3 minutes, then turn. Baste well with whatever is left of the marinade
- Continue to grill, topping the mushroom with the cheese slice for the last 2 minutes. Put to one side and keep warm.
- Split the burger bun, grill the inside till toasted.
- Put your burger stack together. Top the base of the roll with the mushroom (cheese up), then add a good spoon of caramelised onion, then add tomato, then salad greens. If liked, add mustard, ketchup, mayo or a mixture of all three.
- Enjoy!
Nutrition
That’s why I’m calling this idea for a portobello mushroom burger a diet plus recipe. It’s quite nice enough to have on a really low-calorie day, without the bun and being ultra careful with the toppings. But, honestly, would you question this on your dinner plate any day of the week?
Why not save this recipe for later? It’s easy to make and infinitely variable.
This recipe was originally published on London Unattached
Healthy Portobello Mushroom Burger – FAQ
What is this Healthy Portobello Mushroom Burger?
This Healthy Portobello Mushroom Burger is a low-calorie vegetarian burger made with a large portobello mushroom marinated in balsamic vinegar, olive oil and rose harissa, then grilled and topped with caramelised red onion, light cheese, tomato, salad greens and a burger bun.
How many calories are in this Portobello Mushroom Burger?
The recipe serves 1 person and is approximately 308 kcal per burger, including the bun, cheese and toppings. This includes around 37 g carbohydrates, 12 g protein and 14 g fat, making it a lighter alternative to a traditional meat burger.
What ingredients do I need?
You will need one large portobello mushroom, a thin slice of light Leerdammer (or similar cheese), balsamic vinegar, olive oil, rose harissa, a medium red onion, a slice of tomato, low-calorie mayonnaise, salad greens and a burger bun. The mushroom is marinated, grilled and then stacked with the toppings in the bun.
How do I cook the portobello mushroom for the burger?
Mix the balsamic vinegar, olive oil and rose harissa to make a marinade, then baste the mushroom and leave it for about 20 minutes. Grill the mushroom on a griddle, barbecue or under the grill, starting cap-side up for 2–3 minutes, then turn, baste again and continue to cook, adding the cheese for the last couple of minutes so it melts over the mushroom.
Can I make this Portobello Mushroom Burger in advance?
No, this burger is best cooked and eaten straight away. The grilled mushroom, melted cheese and salad lose their texture if left to sit, so this recipe is not intended to be made in advance or used as a make-ahead meal-prep option.
Can I freeze this Portobello Mushroom Burger?
No, this recipe is not suitable for freezing. The grilled mushroom, bun and fresh salad ingredients do not freeze and defrost well – the texture becomes soggy and unappealing. Enjoy the burger freshly cooked instead of freezing it.
Is this burger suitable for the 5:2 diet or other low-calorie plans?
Yes, this burger was created with the 5:2 diet in mind. The mushroom itself is very low in calories and the finished burger is around 308 kcal, depending on the bun and toppings you use. For an even lighter option, you can omit the bun or use just the base as an open sandwich and keep sauces to a minimum.
Is this Portobello Mushroom Burger vegetarian?
Yes, t
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