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. Cook, stirring frequently, until the onions are deeply caramelized and golden brown, approximately 25 minutes.
03 - Add the minced garlic and thyme to the caramelized onions. Stir and cook for 1 minute until fragrant.
04 - Pour in the dry white wine if using, scraping up any browned bits from the bottom of the skillet. Let the liquid simmer for 2 to 3 minutes to reduce slightly.
05 - Stir the rinsed rice into the onion mixture, coating each grain thoroughly. Cook for 2 minutes, allowing the rice to lightly toast and absorb the flavors.
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 casserole 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 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 - Remove from the oven and let the casserole stand for 5 minutes before serving to allow the flavors to settle.