Begin by bringing a large pot of salted water to a boil. Add the 12 jumbo pasta shells and cook until just al dente, typically around 8-10 minutes (follow package directions). Drain and set aside on a lightly greased baking sheet to prevent sticking.
While the pasta cooks, steam the broccoli florets until bright green and fork-tender, about 4-5 minutes. Use a steamer basket over boiling water or microwave in a covered dish with a splash of water. Once done, chop the steamed broccoli into smaller pieces for easier stuffing.
In a large mixing bowl, combine the shredded cooked chicken, steamed broccoli, ricotta cheese, shredded mozzarella, grated Parmesan, garlic powder, onion powder, salt, and black pepper. Stir everything together until well incorporated and creamy.
Gently fill each cooked pasta shell with a generous spoonful of the cheesy chicken broccoli mixture. Place the filled shells seam side up in a baking dish that’s been spread with a thin layer of marinara sauce.
Pour the remaining marinara sauce evenly over the stuffed shells. Cover the dish tightly with aluminum foil and bake at 375°F (190°C) for 25-30 minutes until heated through and bubbly.
Remove the foil and sprinkle chopped fresh parsley over the top for a burst of color and freshness. Serve warm and enjoy the gooey, cheesy goodness!