Bring a large pot of salted water to a boil. Add the rigatoni and boil 10 minutes, or until it begins to soften. Drain.
Meanwhile, add the olive oil to a large saute pan or Dutch oven. Crumble in the sausage and cook until browned, 4-5 minutes. Remove from the pan. Add the eggplant and cook until softened, 5-8 minutes. Add the cooked sausage, sauce, and spinach. Stir until the spinach wilts. Stir in the cooked pasta and ricotta cheese.
Divide the pasta between 2 8-inch square baking dishes. Top with shredded mozzarella. Bake 20 minutes, or until the cheese is melted and bubbling. Broil 3-4 minutes, to lightly brown the mozzarella.
Let rest 10 minutes before serving.
If only one pan of pasta is desired, wrap the second one and freeze it instead of baking. When ready to heat, bake at 350ºF for 60 minutes, or until warmed through.