Preheat your oven to 400°F (200°C).

Wash and chop the Yukon Gold potatoes into 1-inch chunks. Dice bell peppers and onions, and mince the garlic.

In a large skillet, heat a couple of tablespoons of olive oil over medium-high heat. Add the Italian sausage, breaking it into smaller pieces. Sauté for about 6-8 minutes until browned. Remove from the skillet, leaving drippings.

In the same skillet, add the diced onions and sauté for 3-4 minutes until translucent. Add bell peppers for 3-4 minutes, then stir in the minced garlic for 1 minute.

Sprinkle the sautéed mixture with salt, pepper, Italian seasoning, and optional smoked paprika or red pepper flakes. Stir in marinara sauce if using.

In a separate bowl, drizzle chopped potatoes with olive oil and season with additional salt and pepper. Toss well and combine in the skillet with the vegetables and sausage.

Pour the mixture into a greased baking dish. Cover with aluminum foil and bake for 35-40 minutes until potatoes are tender.

Remove foil, stir the casserole, and sprinkle mozzarella and Parmesan cheeses over the top. Return to oven for an additional 10-15 minutes until cheese is melted.

Optionally, broil for 2-3 minutes for extra crispiness, watching closely.

Allow casserole to rest for 5 minutes before serving. Sprinkle with fresh parsley if desired.