Preheat your oven to 375°F (190°C).

Cook the spinach in a skillet with a touch of olive oil over medium heat until wilted, about 3-4 minutes.

Combine the ricotta, cooked spinach, Parmesan, egg, and salt in a mixing bowl. Stir well until everything is evenly incorporated.

Fill the manicotti shells carefully with the ricotta and spinach mixture using a piping bag or a small spoon.

Prepare the marinara sauce by heating olive oil in a saucepan over medium heat. Sauté minced garlic for about 1 minute, then add crushed tomatoes, oregano, salt, and pepper. Simmer for 10 minutes.

Layer half of the marinara sauce on the bottom of a baking dish. Place the filled manicotti shells on top, then pour the remaining marinara sauce over the shells.

Bake in the preheated oven for about 25-30 minutes, or until the sauce is bubbling and the manicotti is tender.

Garnish with fresh basil before serving.