Depending on the cut of beef liver, the membrane may need to be removed if it's still attached. You can do this easily by using a paring knife to cut through the membrane and peel off the membrane.
Gently rinse liver in water. Then, place liver in a shallow bowl and cover with milk. Let liver soak in milk for at least 30-minutes and up to 2 hours. This will remove the bitter flavor. Remove from milk, rinse in cold water, and pat dry with paper towels.*
Thinly slice liver into 1/2-3/4″ thick slices, and slice yellow onion into rings.
In a large bowl, mix the flour with the garlic powder, salt and pepper. Place sliced liver in flour mixture and coat evenly on both sides.
Melt 2 Tbps butter in a large skillet. Add the individual onion rings to skillet and sauté until they’re soft on medium-high heat. Remove the onions, and set aside on another plate.
Place coated liver into skillet with butter. Cook on medium-high heat until golden brown. It will be about 2 minutes per side.
Add onions back to skillet with liver. Reduce heat to medium. Cook until onions are heated and liver is slightly pink in center, approximately 3 minutes. Remove liver and onions from skillet and transfer from a plate.
Notes
*If dairy-free, soak liver in water with two tablespoons of apple cider vinegar or lemon juice.