Liver with Balsamic Vinegar

In large skillet, heat half of the oil over medium heat; fry onions, garlic, half each of the salt and pepper and the thyme, stirring occasionally, until softened, about 8 minutes. Transfer to bowl.

Meanwhile, in shallow dish, combine flour and remaining salt and pepper. Peel membrane from liver; trim off any tough blood vessels. Dredge liver in flour mixture to coat, shaking off excess.

In skillet, heat remaining oil over medium-high heat; fry liver, in batches, turning once and adding more oil if necessary, until slightly pink inside, about 3 minutes. Transfer to plate; keep warm.

Wipe out skillet; add stock and vinegar and bring to boil. Return onion mixture to pan and heat through, about 2 minutes. Stir in parsley and butter; cook until slightly thickened, about 1 minute. Serve over liver.