Bring a large pot of salted water to a boil and cook the jumbo pasta shells until al dente according to the package instructions; drain and set aside.
Preheat the oven to 400°F (200°C) and position a rack in the middle.
In a mixing bowl, combine shredded cooked chicken, softened cream cheese, lemon juice (about 1 tbsp), garlic powder, grated Parmesan, and salt and pepper to taste; stir until smooth and well blended.
Lightly grease an oven-proof skillet or baking dish with 1/2 tablespoon olive oil to prevent sticking.
Spoon the chicken mixture into each cooked shell using a teaspoon or small spoon until shells are filled.
Arrange the filled shells in the prepared skillet or baking dish in a single layer, then sprinkle the grated mozzarella evenly over the top.
Bake for 20–25 minutes, until the cheese is melted and the dish is heated through; if desired, broil for a few minutes to brown the cheese, watching carefully to avoid burning.
Remove from the oven and serve immediately.
