Rinse the beef liver and pat it dry
Trim away any membranes, veins, or discolored spots
Slice the liver into even pieces
Soak the slices in milk or salted water for 30 to 60 minutes if desired
Drain and pat dry again
Season with salt and pepper
Dredge lightly in flour if desired
Heat a skillet over medium heat with butter, oil, or both
Add sliced onions if desired and cook until softened
Place the liver in the hot skillet in a single layer
Cook for 2 to 4 minutes per side, depending on thickness
Avoid overcooking
Remove from the pan when just cooked through
Serve immediately with onions or your preferred side dishes