Preheat your oven to 375°F (190°C).
If using shelf-stable gnocchi, they’re ready to go. For frozen gnocchi, give them a quick boil according to package instructions.
In a large oven-safe skillet, heat olive oil over medium heat. Sauté the garlic until fragrant, about 1 minute. Add the sausage or protein of choice (if using) and cook until browned. Set aside.
In the same skillet, mix the marinara sauce with the heavy cream and dried oregano.
Start layering the casserole: Add a bit of the sauce mixture, followed by gnocchi, spinach, tomatoes, cooked sausage, and a sprinkle of mozzarella and Parmesan cheese. Repeat layers as needed until the ingredients are used or you reach the skillet’s top.
Finish with a generous layer of mozzarella and Parmesan on top.
Bake for 20-25 minutes, until the cheese is bubbly and golden brown. For a crispier top, broil for an additional 2-3 minutes.
Sprinkle with fresh basil leaves before serving.
Let the casserole rest for 5-10 minutes before serving for the best consistency.