01 - Preheat oven to 375°F.
02 - In a large skillet, heat olive oil and butter over medium-low heat. Add sliced onions with a pinch of salt and cook, stirring frequently, until deeply caramelized and golden brown, about 25 minutes.
03 - Add minced garlic and thyme to the caramelized onions, cooking for 1 minute until fragrant.
04 - Deglaze the skillet with white wine if using, scraping up any browned bits from the bottom. Let simmer for 2 to 3 minutes to reduce slightly.
05 - Stir in the rinsed rice, coating each grain with the onion mixture, and cook for 2 minutes to lightly toast.
06 - Transfer the onion-rice mixture to a greased 2-quart 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 shredded Gruyère and grated Parmesan evenly over the top, then scatter the toasted bread croutons. Bake uncovered for an additional 10 to 12 minutes until the cheese is melted and bubbling.
09 - Let the casserole stand for 5 minutes before serving to allow the flavors to settle.