Prepare the pasta according to package directions. Simmer the browned ground beef with the spaghetti sauce as the pasta cooks.
Mix the ricotta, Parmesan, egg, spinach, salt and pepper together.
Place enough sauce in the bottom of your microwaveable dish to barely cover the bottom. Add a layer of drained ziti. Dot the ziti with tablespoons of ricotta mixture. Smooth into a layer. Sprinkle about ½ cup of mozzarella cheese. Add a layer of meat sauce.
Repeat layers, omitting the mozzarella layer and ending with meat sauce. Top with remaining mozzarella.
Place the casserole into the microwave. Cover and set for 15 to 20 minutes on high. Allow the casserole to rest inside the microwave for about two minutes.