Bring a large pot of salted water to a boil over high heat. Add ziti and cook until al dente. Drain in a colander.
In a large skillet over medium heat, saute turkey sausage. Add onion and garlic and saute until the sausage is cooked through. Use your spoon to break up the sausage while it cooks. Add the can of diced tomatoes and pesto and let simmer for 10 minutes.
Add the ricotta cheese, spinach, Parmesan and mozzarella to a large bowl, and stir to combine.
Butter a 4-quart baking dish, add the cooked pasta, then sausage mixture and cheese mixture. Top with a sprinkling of mozzarella and Parmesan.
Bake until completely heated through and golden and crisp on top, about 20 minutes.