Rinse chicken livers under cold water, then place them in a colander. Remove any visible connective tissue or greenish spots.
Heat olive oil in a wide pan over medium heat. Add the sliced onion and cubanelle pepper. Cook for 4–5 minutes until softened and lightly golden.
Add the chicken livers to the pan. Cook for 2–3 minutes as their natural liquid releases.
Flip the livers and cook another 2 minutes, allowing most of the liquid to evaporate.
Stir in the garlic, paprika, and black pepper (and chili flakes if using). Cook for about 30 seconds until fragrant.
Add the tomato sauce and salt. Lower the heat to medium-low and let everything simmer gently for 5 - 6 minutes covered, just until the livers are cooked through and coated in the sauce.
Check doneness: the thickest part of the livers should reach 165°F (74°C) on an instant-read thermometer.
Sprinkle with fresh parsley and serve. Bon appétit.