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A gourmet powerhouse of a burger that swaps the heavy wheat bun for two large, juicy roasted Portobello mushroom caps. Featuring a high-quality beef patty topped with tangy, melted blue cheese and a hit of sharp red onion, this meal delivers the ultimate "burger night" experience while maintaining a negligible impact on blood glucose.
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Prep Time 15 minutes
Cook Time 20 minutes
Course Main Course
Servings 2
Calories 707 kcal

Equipment

  • Large frying pan or griddle
  • Small baking tray
  • Sharp knife
  • Chopping board
  • Spatula

Ingredients
  

  • 4 large Portobello mushrooms (stalks removed)
  • 300 g high-quality ground beef 20% fat
  • 1 tbsp olive oil
  • 1 tsp salt
  • ½ tsp black pepper
  • 60 g Blue cheese Stilton or Gorgonzola
  • 30 g Mayonnaise full fat
  • ½ small red onion finely sliced
  • 20 g Rocket arugula
  • 1 tsp Dijon mustard

Instructions
 

  • Preheat your oven to 200°C.
  • Place the Portobello mushroom caps on a baking tray, gill-side up. Drizzle with a little olive oil and a pinch of salt. Roast for 12–15 minutes until tender but not collapsed. Pat dry with paper towels to remove excess moisture.
  • Divide the ground beef into two 150g portions and shape into wide, flat patties (slightly wider than the mushrooms, as they will shrink). Season both sides generously with salt and pepper.
  • Heat a frying pan over high heat with 1 tbsp of olive oil. Sear the patties for 3–4 minutes on the first side.
  • Flip the burgers. Immediately crumble the blue cheese over the seared side and cover the pan with a lid for 2–3 minutes to melt the cheese while the burger finishes (medium-rare to medium).
  • To assemble, place one mushroom cap (gill-side up) as the base.
  • Spread a little Dijon mustard and mayonnaise inside the mushroom cap.
  • Add a handful of rocket and the sliced red onion.
  • Place the blue cheese-covered patty on top.
  • Finish with the second mushroom cap as the "top bun."
  • Serve immediately with a knife and fork or extra napkins!

Notes

Why This Works for Keto

The classic burger is often a keto minefield due to the bun and hidden sugars in sauces. This Ketropics adaptation completely removes the grain-based carbohydrate source by using Portobello mushrooms, which provide a rich "umami" flavour and a satisfying, meat-like texture with minimal net carbs. The switch from a wheat bun to a mushroom bun alone saves approximately 30-40g of carbohydrates.
By selecting 20% fat ground beef and pairing it with whole-fat blue cheese and mayonnaise, we ensure the meal is fat-dominant. This stimulates the release of cholecystokinin (CCK), a hormone that signals satiety to the brain, helping you stay full for longer. The high protein content supports muscle maintenance, while the blue cheese provides a burst of flavour without the need for sugary ketchups or relishes. This is a perfect example of a meal that supports effortless weight management while feeling like a true indulgence.

Full Ingredients + Nutrition Table

Ingredient
Qty
Calories
Total Carbs
Fibre
Net Carbs
Sugars
Fat
Sat Fat
Protein
Ground Beef (20%)
300g
750 kcals
0.0g
0.0g
0.0g
0.0g
60.0g
24.0g
52.0g
Portobello Caps
4 (approx 280g)
62 kcals
9.2g
3.6g
5.6g
4.8g
1.0g
0.2g
6.0g
Blue Cheese
60g
210 kcals
1.4g
0.0g
1.4g
0.3g
17.4g
11.2g
12.8g
Mayonnaise
30g
204 kcals
0.2g
0.0g
0.2g
0.2g
22.5g
3.5g
0.3g
Red Onion
30g
12 kcals
2.8g
0.5g
2.3g
1.3g
0.1g
0.0g
0.3g
Olive Oil
15ml
133 kcals
0.0g
0.0g
0.0g
0.0g
15.0g
2.0g
0.0g
Rocket
20g
5 kcals
0.7g
0.3g
0.4g
0.4g
0.1g
0.0g
0.5g
Dijon Mustard
5g
5 kcals
0.3g
0.2g
0.1g
0.1g
0.3g
0.0g
0.3g
Seasoning
—
33 kcals
0.0g
0.0g
0.0g
0.0g
0.0g
0.0g
0.0g

Nutrition Facts

Serving size: 1 serving
Servings per container: 2
Amount per serving
Value
% Daily Value*
Calories
707 kcals
35%
Fat
58.2g
83%
– Saturated Fat
20.4g
102%
– Polyunsaturated Fat
6.4g
—
– Monounsaturated Fat
28.1g
—
– Trans Fat
0.0g
—
Cholesterol
145mg
48%
Sodium
1,180mg
51%
Total Carbohydrate
7.3g
18%
– Dietary Fibre
2.3g
9%
– Total Sugars
3.6g
60%
– Net Carbs
5.0g
13%
Protein
36.1g
72%
*Percent Daily Values based on a 2,000 kcal diet with an upper limit of 40g of Net / Total Carbs and 6g of Sugars.