Rinse the beef liver slices under cold water and pat them dry with paper towels.
In a shallow dish, combine the flour, salt, black pepper, and garlic powder. Dredge each slice of liver in the flour mixture, shaking off any excess.
Heat the vegetable oil in a large skillet over medium heat. Once hot, add the sliced onions and sauté for about 5-7 minutes, or until they are soft and golden brown. Remove the onions from the skillet and set aside.
In the same skillet, add the butter and let it melt. Add the coated liver slices and cook for about 3-4 minutes on each side, or until browned and cooked through. Avoid overcooking, as liver can become tough.
Return the sautéed onions to the skillet with the liver, stirring to combine. Cook for an additional minute to heat through.
Serve immediately, garnished with fresh parsley.