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 thinly sliced onions with a pinch of salt. Cook, stirring frequently, until deeply caramelized and golden brown, approximately 25 minutes.
03 - Add minced garlic and thyme to the caramelized onions. Stir and cook for 1 minute until fragrant.
04 - Pour in the dry white wine, scraping up any browned bits from the bottom of the pan. Let simmer for 2–3 minutes to reduce slightly.
05 - Stir in the rinsed rice, coating 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 casserole dish tightly with aluminum foil and bake for 30 minutes until the rice has absorbed 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 10–12 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.