Step 1: Preheat Oven, Cook Sausage, Aromatics & Spinach: Preheat your oven to 375°F. This high temperature ensures the cheese gets beautifully golden and bubbly on top while the gnocchi cooks through perfectly. Heat olive oil in a large oven-safe skillet over medium-high heat. Add the Italian sausage, breaking it up with a wooden spoon into bite-sized crumbles. Cook for 6-8 minutes until browned and cooked through with crispy edges. The caramelization here is key for flavor, so don't rush this step. Reduce to medium heat and add the diced onion to the pan with the sausage and cook for 3-4 minutes until softened and translucent. Add the minced garlic, Italian seasoning, kosher salt, black pepper, and red pepper flakes (if using), and cook for another 30 seconds until fragrant. Don't let the garlic burn.
Step 2: Build the Sauce: Pour in the crushed tomatoes or tomato sauce and water, stirring to combine all the ingredients. Bring the mixture to a gentle simmer for 5 minutes.
Step 3: Add Gnocchi & Basil: Add the gnocchi directly to the sauce (no need to pre-cook!) and stir gently to coat each piece. Sprinkle ribbons of basil over the pasta and sauce.
Step 4: Add the Cheese: Remove the pan from the heat. Using a 1.5 tablespoon cookie scoop or a spoon, dollop in ricotta cheese, dispersing it evenly around the skillet. Then sprinkle the mozzarella cheese, then the parmesan in an even lay over the top of the gnocchi mixture, covering as much surface area as possible for maximum melty, cheesy goodness.
Step 5: Bake, Rest & Serve Hot: Transfer the skillet to your preheated oven and bake for 30 minutes, until the cheese is melted, golden, and bubbly, and the gnocchi are tender. If you want extra browning on top, turn on the broiler for the last 1-2 minutes, watching carefully to prevent burning. (*I put my skillet on top of a baking sheet lined with aluminum foil for easy clean up, just in case the skillet bubbles over.) Remove from the oven and let it rest for 5 minutes. This allows the sauce to thicken slightly and makes serving easier. Garnish with fresh basil and additional Parmesan if desired. Serve hot and enjoy!