Fry bacon until crisp. Drain on paper towels and reserve.
Melt 4 tablespoons butter in a medium skillet over moderate heat. Cook shallots with salt until soft and nearly golden, about 3 minutes. Add cooked bacon, stirring briefly to rewarm. Add vinegar, turn the heat to high and reduce by half. Add stock and reduce again by half. Break remaining butter into small pieces and whisk into sauce until smooth. Whisk in mustard and remove from heat.
Preheat grill or a dry skillet. Season liver with salt and pepper. Grill or saute in melted butter over high heat, about 3 minutes per side.
Arrange liver over a bed of Sauteed onions on individual serving plates and spoon on sauce. Serve immediately.
Melt butter in a large skillet over high heat. Reduce heat. Saute onions with salt, stirring occasionally, until golden brown, about 15 minutes.
c.1997, M.S. Milliken & S. Feniger, all rights reserved