Dredge liver in flour. In a large skillet; heat oil and butter. Add liver and cook until almost cooked on both sides. Place liver in casserole dish and keep warm.
Add onions to skillet. Sauté until translucent. Add brown sugar, vinegar, soy sauce and beef broth. Bring to a boil. Mix corn starch with bit of water and slowly blend into onion mixture. Cook for 2 minutes. Pour sauce over liver in casserole dish and bake in preheated 350 F (180 C) oven for 1 hour.