Preheat your oven to 350°F (175°C).

In a large pot, bring salted water to a rolling boil. Add the jumbo shells one at a time. Boil until slightly less than al dente. Use a slotted spoon to remove them and place on a cutting board to cool.

In a medium skillet, heat olive oil over medium heat. Add minced garlic and cook for about 30 seconds until fragrant. Add the baby spinach, sautéing until wilted (about 2 minutes). Transfer to a large bowl.

In the large bowl, add shredded chicken, ricotta cheese, cubed mozzarella, ¼ cup of Parmesan, egg, kosher salt, and black pepper. Mix well until fully combined.

In the same skillet, combine the marinara sauce and heavy cream, bringing it to a boil. Reduce heat to simmer and stir in the remaining Parmesan, allowing it to melt.

Pour the prepared sauce into a 9x12-inch baking dish, spreading it evenly. Stuff each cooled jumbo shell with the filling and place in the dish. Top with shredded mozzarella.

Bake uncovered for 25 to 30 minutes, until bubbly and golden brown.

Let cool slightly, garnish with fresh basil if desired, then serve warm.