Prepare the liver: Rinse liver under cold water and pat dry. Slice into thin pieces (about 1 cm thick).
Marinate (optional): Sprinkle with vinegar or lemon juice and let sit for 10–15 minutes to remove strong odor and tenderize.
Cook onions and garlic: Heat oil or butter in a pan over medium heat. Sauté onions until translucent, then add garlic and chilies. Cook 1–2 minutes.
Cook the liver: Increase heat to medium-high. Add liver slices to the pan in a single layer. Cook 2–3 minutes per side — do not overcook, or liver becomes tough and grainy.
Season: Sprinkle with salt, pepper, and paprika (if using). Stir gently.
Serveren: Remove from heat, garnish with fresh parsley, and serve immediately.