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 along with a pinch of salt. Cook, stirring frequently, until the onions are deeply caramelized and golden brown, approximately 25 minutes.
03 - Stir in the minced garlic and thyme. Cook for 1 minute until fragrant.
04 - Pour in the white wine, scraping up any browned bits from the bottom of the skillet. Let simmer for 2–3 minutes to reduce slightly.
05 - Add the rinsed rice to the onion mixture, stirring to coat each grain. 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 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 shredded Gruyère and grated Parmesan evenly over the top. Scatter the toasted bread croutons across the surface. Bake uncovered for an additional 10–12 minutes until the cheese is melted, golden, and bubbling.
09 - Remove from the oven and let stand for 5 minutes to allow the flavors to settle before serving.