Creamy, garlicky ricotta stuffed into jumbo shells with tender chicken and fresh spinach, nestled in marinara and baked under a blanket of gooey mozzarella for cozy Italian-American comfort.
Preheat the oven to 375°F (191°C). Lightly grease a 9×13-inch (about 3-quart) baking dish and set aside.
Bring a large pot of salted water to a boil. Add the jumbo shells and cook until just al dente, about 9 minutes. Drain, rinse with cold water, and arrange shells open-side up on a baking sheet.
Heat the olive oil in a large skillet over medium heat. Sauté the minced garlic for 30 seconds, then add the spinach and cook, stirring, until wilted, about 3 minutes. Remove from heat.
In a large bowl, combine ricotta, salt, pepper, Italian seasoning, egg, grated parmesan, and 1 cup (113g) of the mozzarella.
Fold in the cooked chicken and the garlic–spinach mixture until evenly combined.
Spread about 2 cups (about 450g) of marinara sauce in the bottom of the prepared baking dish.
Spoon the ricotta–chicken filling evenly into the shells and arrange them in the baking dish.
Pour the remaining marinara sauce over the shells and sprinkle with the remaining mozzarella cheese.
Bake, uncovered, for 20–25 minutes, until bubbly and lightly golden.
Garnish with fresh basil and extra parmesan for serving, if desired.