Onion Au Gratin Recipe

Preheat oven to 375°F (190°C). Thinly slice onions so they measure about 6 cups. In a large heavy skillet or sauté pan, heat 1 tablespoon olive oil and 2 tablespoons butter over medium heat until shimmering.

Add the sliced onions, 1 tsp salt and 1 tsp freshly ground black pepper. Cook, stirring every few minutes, until the onions are deeply golden and soft, about 25–35 minutes. Reduce heat if onions begin to burn; you want slow caramelization to develop sweet, savory flavor (this creates the gratin's core flavor).

While onions cook, in a separate small skillet over medium heat, add diced bacon (6 oz). Cook until the fat renders and bacon is crisp-tender, about 6–8 minutes. Using a slotted spoon, transfer bacon to a paper towel-lined plate, leaving about 1 tablespoon of bacon fat in the pan.

Add the minced garlic to the bacon fat and sauté 30–45 seconds until fragrant. Remove from heat and set aside with the bacon.

When onions are caramelized, push them to one side of the pan. Increase heat to medium-high. Pour in the dry white wine (1 tbs) to deglaze, scraping up browned bits from the pan for about 1 minute to concentrate flavor.

Stir in the beef demi-glace (1 cup), chicken or beef broth (1 cup) and 1 teaspoon Worcestershire sauce. Add the thyme sprigs. Bring to a gentle simmer and cook 5–7 minutes until the liquid reduces slightly and becomes glossy and full-bodied. Taste and adjust seasoning with additional salt or pepper if needed. Remove thyme sprigs.

If you prefer a slightly thicker jus for the gratin, sprinkle 1 teaspoon all-purpose flour evenly over the onions and stir to combine, cooking 1–2 minutes before adding the liquid. This will help the mixture bind when baked.

Reduce heat to low and stir in the heavy cream (1 cup) until fully incorporated and the mixture is slightly thickened, about 2–3 minutes. Remove from heat. Fold in half of the grated gruyere (about 1/2 cup) so it begins to melt and create a rich, cheesy base.

Stir the cooked bacon and sautéed garlic into the onion-jus-cream mixture, reserving about 2 tablespoons of bacon for finishing if you want a crisp topping.

Butter a 9x6-inch (or similarly sized) ovenproof gratin dish or individual ramekins with the remaining 1 tablespoon butter. Transfer the onion, bacon and soupy-cream mixture into the dish, spreading evenly.

Combine the remaining grated gruyere (1/2 cup) with parmesan (1 tbs) and optional breadcrumbs (2 tbs). Evenly sprinkle this cheese (and breadcrumb) mixture over the top. Dot with the reserved crisp bacon pieces.

Bake in the preheated oven for 15–20 minutes, until the top is bubbly and golden-brown. For a deeper gratin crust, place under the broiler for 1–2 minutes watching closely to avoid burning.

Remove from oven and let rest 5 minutes. Sprinkle with chopped chives (1 tbs) for color and freshness before serving.