Preheat the oven to 425F.

Make the spinach & ricotta: Heat the olive oil in a 12-inch oven-safe skillet (it needs to be at least this big; otherwise use an oven-safe pot instead) over medium heat. Add the spinach and garlic, and a sprinkle of salt and pepper. Sauté until wilted, 2-3 mins. Transfer to a bowl and stir in the ricotta, ¼ tsp kosher salt, and a few grinds of pepper, then set aside in the fridge.

Make the ziti: In the same skillet, heat the olive oil over medium heat. Add onion and cook until softened, 3-4 mins. Add the garlic and cook 30 seconds, then add the sausage. Cook, breaking it into small pieces with your spatula, until browned, about 5-6 mins.

Stir in the Italian seasoning, red pepper flakes (if using), sugar, and salt. Stir in the marinara sauce, then stir in the uncooked pasta. Add the water.

Bring to a simmer, then reduce the heat to medium-low and cook uncovered, stirring occasionally, until the pasta is tender and the sauce thickens, 12-15 mins. (You can add more water if you'd like the pasta more tender once the excess liquid is absorbed, but I didn't have to.)

Spoon dollops of the spinach ricotta mixture evenly across the top, then sprinkle with shredded mozzarella.

Transfer the skillet to the oven and bake for 15-20 mins, until the cheese is melted, golden brown, and some of the pasta edges are crisped. Serve warm with grated parm on top and enjoy!