Soak the beef liver in buttermilk for at least 1 hour to remove bitterness and tenderize the meat.
Remove the liver from the buttermilk, pat dry, and slice into 1/2 inch thick pieces.
Combine the flour, salt, and black pepper in a shallow dish. Dredge the liver pieces in the flour mixture.
Heat olive oil in a large skillet over medium heat. Add butter and allow it to melt.
Sauté sliced onions for about 5 minutes, then add minced garlic and cook for 1 more minute. Set aside.
Cook the dredged liver pieces in the skillet for 2-3 minutes on each side until browned and cooked through.
Return the sautéed onions and garlic to the skillet, stir to combine, and heat through for another minute before serving.
