1. Rinse liver slices under cold water and pat dry with paper towels.
2. In a shallow dish, combine flour, salt, and pepper. Dredge liver slices in the flour mixture.
3. In a large skillet, heat butter and olive oil over medium heat. Add onions and cook until soft and golden brown, about 10 minutes. Remove onions from skillet and set aside.
4. In the same skillet, add liver slices and cook over medium heat until browned on both sides, about 3 minutes per side.
5. Return onions to the skillet, reduce heat to low, and simmer for another 5 minutes. Serve hot.
Notes:
For best results, soak liver in milk for 30 minutes before cooking to reduce bitterness.