Preheat your oven to 350°F (175°C) and grease a 9x13 casserole dish with olive oil or cooking spray.

In a large skillet over medium heat, add olive oil and Italian sausage, breaking it into small chunks. Cook for about 8-10 minutes until golden brown and cooked through. Stir in Italian seasoning and rosemary, then transfer to a plate.

In the same skillet, add diced onion and red bell peppers. Sauté for about 6 minutes until soft, then add spinach, minced garlic, salt, and pepper, cooking for an additional 2 minutes.

Combine the sausage and sautéed vegetables in the skillet.

In a large mixing bowl, whisk together eggs, full-fat milk, ricotta cheese, and grated Parmesan cheese until well combined.

Layer half of the sausage and vegetable mixture into the greased baking dish, add half of the egg mixture, then repeat with the remaining sausage, veggies, and egg mixture.

Cover and refrigerate overnight or bake immediately for about 55 minutes until set and golden brown.

Once baked, let the casserole cool for about 10 minutes before cutting into squares and serving.