Preheat oven to 375°F (190°C). Lightly grease a baking dish and set aside.
Bring a large pot of salted water to a boil. Cook fettuccine according to package directions until al dente. Reserve ¼ cup pasta water, then drain.
In a large bowl, combine marinara sauce, heavy cream, Italian seasoning, garlic powder, salt, and black pepper.
Add cooked chicken and drained fettuccine to the sauce mixture. Toss until evenly coated. Add reserved pasta water if needed to loosen the sauce.
Transfer the pasta mixture to the prepared baking dish.
Sprinkle mozzarella and Parmesan evenly over the top.
Bake uncovered for 20–25 minutes, or until bubbly and lightly golden
Remove from oven and let rest for 5 minutes before serving. Garnish with fresh herbs if desired.