01 - Preheat oven to 375°F. 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, approximately 25 minutes.
03 - Stir in the minced garlic and thyme leaves, cooking for 1 minute until fragrant.
04 - Pour in the dry white wine if using, scraping up any browned bits from the bottom of the pan. Let simmer for 2 to 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 to lightly toast the rice.
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 bread croutons, and 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.