Start by peeling, seeding, and dicing the squash into cubes. Preheat your oven to 375°F (190°C). Boil the squash for about 10 minutes until tender, then drain and set it aside.
Heat some olive oil in a skillet over medium heat. Sauté the onions and garlic until they’re fragrant and translucent. This step adds a flavorful base to the casserole.
Toss in the chopped bell peppers and sauté until soft. Next, stir in the spinach and cook for a minute until it wilts. These colorful veggies pack the dish with nutrients and flavor.
Combine the cooked squash, sautéed veggies, shredded cheddar, Parmesan, and breadcrumbs in a mixing bowl. This hearty mix is going to be the core of your casserole.
Whip together heavy cream, eggs, thyme, rosemary, and a pinch of salt and pepper. This custard binds the ingredients together and turns your casserole creamy and delicious.
Grease your casserole dish and pour in the squash mixture. Top it with the custard evenly. Bake for about 35 minutes until it’s beautifully golden and bubbling. Don’t forget to savor the heavenly aroma while it bakes!