Equipment
Ingredients
Base
- 600 g yellow onions, peeled and sliced into thin half-moons
- 30 g unsalted butter, room temperature
- 15 ml olive oil
- 2 garlic, minced
- 2 g fresh thyme, leaves stripped and finely chopped
Sauce and Topping
- 150 ml heavy cream
- 40 g vegetarian parmesan cheese, freshly grated
- 60 g gruyere cheese, freshly grated
- 3 g kosher salt
- 1 g black pepper, freshly cracked
Nutrition (per serving)
Nutrition is estimated per serving. Nutrients without complete source data are omitted.
Full nutrition
- Calories
- 380 kcal
- Protein
- 11 g
- Carbohydrates
- 17 g
- Fat
- 31 g
- Fiber
- 3 g
- Sugar
- 8 g
- Sodium
- 534 mg
- Saturated Fat
- 17 g
- Trans Fat
- 0 g
- Monounsaturated Fat
- 10 g
- Polyunsaturated Fat
- 2 g
- Cholesterol
- 82 mg
- Potassium
- 294 mg
- Calcium
- 338 mg
- Iron
- 0.6 mg
- Magnesium
- 29 mg
- Phosphorus
- 232 mg
- Zinc
- 1.3 mg
- Vitamin A
- 268 mcg
- Vitamin C
- 13 mg
- Vitamin D
- 0.7 mcg
- Vitamin E
- 1.2 mg
- Vitamin K
- 6 mcg
- Vitamin B6
- 0.3 mg
- Vitamin B12
- 0.4 mcg
- Thiamin
- 0.10 mg
- Riboflavin
- 0.20 mg
- Niacin
- 0.3 mg
- Folate
- 33 mcg
- Water
- 166 g
Sources: USDA FoodData Central
Method
Prepare your ingredients by slicing the onions into even half-moons, mincing the garlic, and freshly grating both cheeses.
Preheat your broiler to high, approximately 260°C or 500°F.
Heat the butter and olive oil in a large oven-safe skillet over medium heat. Add the sliced onions, season with the kosher salt and black pepper, and saute until they are very tender and deeply translucent. This should take about 15 minutes.
Stir in the minced garlic and fresh thyme, cooking for 1 minute until fragrant. Pour in the heavy cream and simmer gently until the liquid reduces and coats the onions thickly, roughly 4 minutes.
Remove the skillet from the stovetop heat. Evenly sprinkle the grated gruyere and vegetarian parmesan over the entire surface of the creamy onions.
Transfer the skillet to the oven and broil for 4 to 5 minutes. Watch closely until the cheese is bubbling, thoroughly melted, and forms a rich golden-brown crust.
Carefully remove the bubbling skillet from the oven. Let the gratin rest at room temperature for 5 minutes before serving to allow the molten cream and cheese to set slightly.
Chef's Notes
- Using a mix of butter and olive oil elevates the flavor while preventing the milk solids in the butter from burning during the extended cooking time.
- Always grate your own cheese for a gratin. Pre-shredded cheeses are coated in starches like cellulose that inhibit smooth melting and can impart a gritty texture to the final dish.
- Traditional Parmigiano-Reggiano contains animal rennet. To keep this recipe strictly vegetarian, you must specifically source a certified vegetarian hard cheese.
- An oven-safe cast iron or heavy stainless steel skillet is essential here to transition seamlessly from stovetop to high-heat broiler without warping or cracking.
Storage
Refrigerator: 3 days — Reheat in the oven to re-crisp the cheese topping
Reheating: Reheat in a 175C/350F oven for 10 to 15 minutes until heated through and bubbling
Troubleshooting
Watery gratin
Cause: Onions were not cooked down sufficiently to release their moisture before adding dairy
Fix: Before adding the cheese topping, simmer the mixture on the stovetop for an additional 2 to 3 minutes until the liquid reduces and properly clings to the onions
Prevention: Ensure onions are completely soft and pan is relatively dry during the initial saute phase
Cream sauce split or looks oily
Cause: Boiling the cream too vigorously or using a low-fat dairy alternative instead of heavy cream
Fix: Remove from heat immediately and briskly stir in a small splash of cold heavy cream to re-emulsify
Prevention: Use genuine heavy cream (at least 35 percent fat) and keep the heat at a gentle simmer
Cultural Context
The gratin technique hails from classical French culinary traditions, originally referring to the scraped, toasted bits clinging to the sides of a cooking pot. Today, it broadly denotes any dish baked with a beautifully browned crust of breadcrumbs or cheese. This onion-centric variation borrows the sweet, deeply savory profile of classic Soupe a l'Oignon and condenses it into a lush, fork-friendly side dish that pairs exceptionally well with roasted vegetables or thick-cut crusty bread.
