Cut bacon into 1 inch pieces. Sauté bacon in a skillet over low heat till fat is translucent, about half way cooked. Drain grease. Add onions. Continue sautéing until onions begin to caramelize. The onions should look "sticky and sugary" and have a golden color. Place onion mixture on a plate and keep warm.

Lightly flour each piece of liver. Over medium-low heat, melt butter in a large skillet and heat till it just begins to smoke. Do not burn the butter. Add liver to skillet and sauté till liver has just a little pink on the inside (cut to check) turning once or twice. Remove liver from skillet and place over the onion mixture.

Add the vinegar to the liver skillet and reduce to about one teaspoon stirring constantly, scraping up brown bits. Add the chicken stock and bring to a boil. Boil the stock till it is reduced to a sauce consistency. Strain the sauce over the liver and serve.