In a medium bowl, combine together ricotta cheese and eggs until blended. Stir in spinach, 2 cups mozzarella cheese, 1/4 cup Parmesan cheese, minced garlic, onion powder, salt and pepper. Transfer mixture to a gallon sized sealable plastic storage bag. Use scissors to cut off the tip from one of the bottom corners of the bag.
Spread about 1-1/2 cups of the marinara sauce in the bottom of a baking dish. Use the makeshift pastry bag to squeeze the spinach and cheese mixture into the uncooked manicotti shells. Arrange the stuffed shells on top of the marinara sauce in a single layer. Pour remaining sauce over the top, being sure that all of the pasta is covered well. Cover the dish with foil and refrigerate for at least 7 or 8 hours or overnight.
Preheat oven to 400 degrees F. Bake covered for 40 minutes. Remove foil, sprinkle with remaining mozzarella and Parmesan cheese, and bake uncovered for another 7 to 10 minutes or util cheese has melted and is beginning to get light golden brown.
Adapted from Allrecipes.com
Options - Add chopped rotisserie chicken or cooked, crumbled Italian sausage to the filling mixture for a meatier dish. You can also scale back on the spinach and add some chopped, fresh basil.