Preheat the oven to 400°F (200°C) and lightly grease a 9x13-inch baking dish.

Heat the olive oil in a large skillet over medium-high heat. Add the sausage slices and cook until browned on both sides, about 5 to 7 minutes. Transfer the sausage to the prepared baking dish.

In the same skillet, add the onion, bell peppers, and mushrooms. Cook for 6 to 8 minutes, stirring occasionally, until the vegetables begin to soften and the mushrooms release their moisture.

Add the garlic, Italian seasoning, crushed red pepper flakes if using, salt, and black pepper. Cook for 1 minute until fragrant.

Pour in the heavy cream and stir in the Parmesan cheese. Simmer for 1 to 2 minutes until slightly thickened, then remove from heat.

Add the vegetable mixture to the baking dish with the sausage and toss gently to combine. Sprinkle 1 cup of the mozzarella evenly over the top.

Bake uncovered for 15 minutes, until hot and bubbly. Remove from the oven, sprinkle with the remaining 1/2 cup mozzarella, and bake for 5 more minutes, or until the cheese is melted and lightly golden.

Let the bake rest for 5 minutes before serving. Garnish with chopped parsley.