Step 1: Start by placing your oven rack in the middle position and preheat your oven to 375 degrees.
Step 2: For the sauce, heat a medium-sized saucepan over medium heat. Once hot, add a drizzle of olive oil and let it warm up. Add the chopped onion and sauté until it’s soft and fragrant, about 5 to 7 minutes.
Step 3: Next, add the minced garlic along with the red pepper flakes and Italian spices. Sauté these for about a minute until the garlic is aromatic, being careful not to let it burn.
Step 4: Pour in the crushed tomatoes and add the tomato paste. Stir everything together, allowing the flavors to meld. Let the sauce simmer for about 20 minutes, stirring occasionally.
Step 5: While the sauce simmers, let’s prepare the filling for the manicotti. In a large mixing bowl, combine the ricotta cheese, 3 cups of your chosen shredded cheese, the egg, salt, and pepper. Mix until everything is well combined.
Step 6: Now, let's prep the noodles. Soak the no-boil lasagna noodles in hot water for about 5 minutes. This will make them pliable and easier to fill.
Step 7: It’s time to assemble! In a 9-inch by 13-inch baking dish, spread half of the sauce on the bottom. Take each noodle and fill it with about 1/4 cup of the cheesy spinach mixture.
Step 8: Spread the remaining sauce over the top of the manicotti. Cover the dish with foil to keep the moisture in while baking.
Step 9: After 40 minutes, remove the foil and sprinkle the remaining 1 cup of cheese over the top. Bake for an additional 5 minutes.
Step 10: Let the manicotti cool for about 10 minutes before serving. This allows the filling to set slightly, making it easier to serve. Enjoy your delicious creation!