Preheat the oven to 400°F. Lightly grease a 9x13-inch baking dish.
Add the uncooked tortellini, chicken, cherry tomatoes, pesto, 1 cup of the shredded mozzarella, ¾ cup water, garlic powder, salt and pepper to the baking dish.
Stir everything together until evenly combined. Make sure the tortellini is well coated and distributed in an even layer.
Cover the baking dish tightly with foil and bake for 20 minutes.
Carefully remove the foil and stir. Check the tortellini for doneness. If the dish looks dry or the tortellini still needs additional cooking, stir in a splash of the remaining water.
Sprinkle the remaining 1 cup mozzarella evenly over the top.
Return to the oven uncovered and bake for another 8–10 minutes, or until the tortellini is tender and the cheese is melted and bubbly.
For a golden top, broil for 1–2 minutes, watching carefully.
Let rest for 5 minutes before scooping and serving.