Equipment
Ingredients
Caramelized Onions
- 2 large yellow onions, thinly sliced
- 30 g unsalted butter
- 15 ml balsamic vinegar
- 2 g fine sea salt
Dijon Aioli
- 80 g mayonnaise
- 15 g dijon mustard, smooth
- 5 ml fresh lemon juice
Portobello 'Patties'
- 4 large Portobello mushroom caps, stems removed, wiped clean
- 4 g fine sea salt
- 2 g black pepper, freshly ground
- 3 g garlic powder
- 30 ml neutral oil
Assembly
- 4 brioche buns, sliced
- 120 g gruyère cheese, sliced or grated
- 15 g unsalted butter, softened
Nutrition (per serving)
Nutrition is estimated per serving. Nutrients without complete source data are omitted.
Full nutrition
- Calories
- 681 kcal
- Protein
- 20 g
- Carbohydrates
- 53 g
- Fat
- 44 g
- Fiber
- 6 g
- Sugar
- 13 g
- Sodium
- 1327 mg
- Saturated Fat
- 15 g
- Trans Fat
- 0 g
- Monounsaturated Fat
- 15 g
- Polyunsaturated Fat
- 13 g
- Cholesterol
- 66 mg
- Potassium
- 802 mg
- Calcium
- 447 mg
- Iron
- 3.5 mg
- Magnesium
- 44 mg
- Phosphorus
- 442 mg
- Zinc
- 2.7 mg
- Vitamin A
- 162 mcg
- Vitamin C
- 10 mg
- Vitamin D
- 0.6 mcg
- Vitamin E
- 2.9 mg
- Vitamin K
- 36 mcg
- Vitamin B6
- 0.5 mg
- Vitamin B12
- 0.6 mcg
- Thiamin
- 0.54 mg
- Riboflavin
- 0.47 mg
- Niacin
- 9.4 mg
- Folate
- 143 mcg
- Water
- 281 g
Sources: USDA FoodData Central
Method
In a small bowl, whisk together the mayonnaise, Dijon mustard, and lemon juice until smooth. Refrigerate until assembly.
Melt 30g butter in a large skillet over medium-low heat. Add sliced onions and 2g salt. Cook, stirring occasionally, until onions are deep golden brown and soft.
Once onions are caramelized, stir in the balsamic vinegar and cook for 1 minute until syrupy. Remove onions and wipe the skillet clean.
Clean the mushroom caps with a damp paper towel. Remove the stems. Brush both sides of the caps with half of the neutral oil.
Mix the salt, black pepper, and garlic powder. Season the mushroom caps generously on both sides.
Preheat the broiler. Heat the remaining oil in the skillet over medium-high heat. Place mushrooms in the skillet, gill-side down. Sear for 4-5 minutes, then flip.
Top each mushroom cap with Gruyère cheese. Transfer the skillet to the broiler for 1-2 minutes until cheese is bubbly and golden.
Remove mushrooms to a plate. Butter and toast the brioche buns in the same skillet until golden.
Spread aioli on the bottom bun. Place the cheesy Portobello cap on top. Finish with a mound of balsamic onions and the top bun.
Chef's Notes
- The Portobello mushroom acts as a naturally savory, meaty base that absorbs the balsamic and garlic flavors perfectly.
- If your mushrooms are very large, you may need to cook them in two batches to avoid crowding the pan.
- To make this recipe vegan, use a plant-based mayonnaise and vegan Gruyère-style cheese.
Storage
Refrigerator: 3 days — Store components separately; mushrooms can be reheated in a pan.
Reheating: Reheat mushrooms in a skillet over medium heat to maintain texture.
Cultural Context
This version adapts the classic French Onion-inspired bistro burger into a vegetarian format by substituting the beef patty with a roasted Portobello mushroom cap, known for its 'steak-like' texture.
