Preheat the oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish.

Bring a large pot of salted water to a boil. Cook jumbo pasta shells just shy of al dente according to package instructions. Drain, rinse with cold water, and spread on a clean towel to prevent sticking.

In a skillet over medium heat, warm a little butter and olive oil. Add minced garlic and cook until fragrant (about 30 seconds). Add chopped spinach and cook until wilted. If using frozen spinach, make sure it’s fully thawed and squeezed-dry before cooking.

In a large bowl, combine the cooked and shredded chicken, cooled spinach, ricotta cheese, shredded mozzarella, grated Parmesan, egg, Italian seasoning, salt, and pepper. Mix until evenly combined.

Spread a thin layer of marinara sauce over the bottom of the prepared baking dish.

Fill each jumbo pasta shell generously with the chicken-ricotta mixture. Arrange the shells in the dish, seam side up.

Cover the shells with the remaining marinara sauce. Sprinkle extra mozzarella on top.

Bake uncovered for 25–30 minutes, until the sauce is bubbling and the cheese is melted and lightly golden.

Let rest for 5 minutes before serving. This helps the stuffed shells set slightly for easier slicing.