Spray a 9x13 baking dish with cooking spray. Set aside. Preheat the oven to 400 degrees Fahrenheit.
Prepare your dairy-free Alfredo sauce and set aside. Cook the penne pasta according to package instructions, then drain and reserve a ¼ cup of pasta water. Set pasta water aside.
While the pasta is cooking, heat the oil on medium in a large saucepan or Dutch oven. Add the onions and cook, stirring occasionally until they start to soften. Then add garlic, stir to combine.
Add the chicken and cook to brown it. Then add the broccoli, salt, pepper, and chili flakes.
Cook until broccoli is warm and chicken is fully cooked.
Stir the reserved pasta water into the Alfredo sauce. Add the pasta followed by the Alfredo sauce to the saucepan. Stir to coat.
Transfer mixture to prepare baking dish. Loosely tent with foil and bake for 10-13 minutes. The top of the pasta bake will be a golden brown and creamy underneath.
Serve warm. Top with ground pepper or red chili flakes.
Notes
*The Dairy-Free Alfredo Sauce can be made the day before. Store in an airtight container in the refrigerator.