Preheat oven to 375 F. Cook pasta to just before al dente, about 6-8 minutes. Drain and set aside.
Meanwhile, heat olive oil in a large saucepan over med-high heat. Add chicken and cook for 3-4 minutes until lightly browned. Add onions and garlic, sauteeing another 1-2 minutes or so until fragrant. Add in marinara sauce, sour cream, salt and pepper and chili flakes, then bring just to a boil, stirring well.
Stir in red pepper and spinach, and then toss pasta in sauce mixture and remove from heat. Add pasta to a large greased 9×13 dish and then top with mozzarella cheese. Bake in the oven for 25-30 minutes until cheese is melted and starts to brown.
Remove from oven, garnish with basil leaves then serve and enjoy! You can freeze individual portions up to 3 months. To reheat, defrost in the microwave for 6-7 minutes or reheat in a 350 F oven for 20-30 minutes.