Prepare the chicken: Season the chicken with salt, pepper, and paprika. Dredge the chicken in flour, shaking off excess.
Cook the chicken: Heat the olive oil in a large skillet over medium-high heat. Add the chicken and cook until browned and cooked through, about 5-7 minutes. Remove the chicken from the skillet and set aside.
Cook the onion and garlic: In the same skillet, add the chopped onion and cook until softened, about 3-4 minutes. Add the minced garlic and cook for an additional minute.
Make the sauce: Add the chicken broth and heavy cream to the skillet, stirring to combine. Bring the mixture to a simmer and cook until the sauce has thickened, about 5-7 minutes.
Add the Parmesan cheese: Stir in the grated Parmesan cheese until melted and smooth. Season the sauce with salt, pepper, and cayenne pepper (if using).
Combine the chicken and sauce: Add the cooked chicken back to the skillet and toss to combine with the sauce.
Serve: Serve the chicken and sauce over cooked spaghetti, garnished with chopped parsley (if desired).