Start by whisking 8 eggs in a large bowl until they are nice and frothy, which should take about 1 minute. Next, add 3 cups of water and 1 teaspoon of salt to help achieve a smoother consistency. Be gentle to avoid overmixing; you want a light batter, not a dense one!
Gradually stir in 3 1/2 cups of flour, combining until there are no dry bits left. The batter should feel thick yet pourable. Let it sit for 10-15 minutes; this helps the flour hydrate fully, which is crucial for making beautiful shells.
Coat an 8-inch non-stick pan with cooking spray, ensuring even coverage, and preheat it over medium heat. Pour some batter into the pan, swirling it for an even layer. Look for the edges to lift slightly; that is your cue to flip! Allow them to cook no longer than 30 seconds per side to avoid rubbery shells.
In another bowl, mix together 1 large container of ricotta, 1 small container of mozzarella, 3 eggs, and 3 tablespoons of parmesan until everything is well blended and creamy. If it feels thick, add a splash of milk or water to lighten it up. Taste a bit to ensure it has the right flavor and consistency.
Grab the cooled pasta shells and stuff each one with the cheesy filling using a large ladle. Place them carefully in a baking dish. Avoid over-stuffing as it can cause them to burst while baking.
Pour 6 cups of marinara sauce over the filled shells, ensuring they are nicely covered. Sprinkle with 1/4 cup of grated parmesan on top for an added layer of flavor.
Pop your baking dish in the oven and bake at 375 degrees F until bubbly and golden, approximately 20-30 minutes. The aroma will fill your kitchen. To prevent sogginess, ensure you do not overbake.
Once baked, let the manicotti sit for about 10 minutes before serving. This helps everything set up nicely, making it easier to slice without the filling oozing out.