Preheat the oven to 400°F (200°C).
Heat 1 tablespoon olive oil in a large skillet over medium heat.
Add the cubed chicken and cook, stirring occasionally, until browned and cooked through, about 5–7 minutes; remove with a slotted spoon and set aside.
Bring a large pot of salted water to a boil, add the jumbo shells, and cook until al dente, about 10 minutes; drain in a colander and let shells cool slightly so they can be handled without tearing.
In a medium bowl, combine the cooked chicken, 15 ounces ricotta, three-quarters of the shredded Parmesan (reserve the rest for topping), and 1 tablespoon Italian seasoning; stir until well mixed.
Spoon a thin layer of marinara sauce into the bottom of a 9×13-inch baking pan.
Fill each shell with the chicken-ricotta mixture and arrange the filled shells in the prepared baking dish.
Pour the remaining marinara sauce evenly over the filled shells and sprinkle the remaining Parmesan cheese on top.
Bake in the preheated oven until heated through and cheese is melted, about 20 minutes; remove from oven and serve.