01 - Preheat oven to 375°F. Lightly grease a 2-quart casserole dish and set aside.
02 - In a large skillet, heat olive oil and butter over medium-low heat. Add the sliced onions with a pinch of salt and cook, stirring frequently, until deeply caramelized and golden brown, about 25 minutes.
03 - Stir in the minced garlic and thyme, cooking for 1 minute until fragrant.
04 - Pour in the white wine, scraping up any browned bits from the bottom of the pan. Let simmer for 2–3 minutes to reduce slightly.
05 - Add the rinsed rice to the skillet, stirring to coat each grain with the onion mixture. Cook for 2 minutes, allowing the rice to lightly toast.
06 - Transfer the onion-rice mixture to the prepared casserole dish. Pour in the broth, season with salt and pepper, and stir gently to combine.
07 - Cover the dish tightly with aluminum foil and bake for 30 minutes until the rice has absorbed most of the liquid.
08 - Remove the foil and fluff the rice with a fork. Sprinkle the Gruyère and Parmesan cheeses evenly over the top, scatter the toasted croutons, and bake uncovered for an additional 10–12 minutes until the cheese is melted and bubbling.
09 - Remove from the oven and let stand for 5 minutes before serving to allow the flavors to settle.