Heat butter in a pan over medium-low heat. Add onions, bay and season the onions with thyme, salt and pepper. Cook to caramel in color and very soft, 40-45 minutes, stirring occasionally. Sprinkle caramelized onions with flour, cook 1 minute then stir in sherry and deglaze pan. Add beef consommé and simmer gently for 5 minutes, until a very thick sauce forms.
Meanwhile, heat a cast-iron skillet or griddle over medium-high heat. Pat steaks very dry and drizzle pan with oil. Rub a paper towel over bottom of the pan to make sure the oil coats the whole surface and to remove any excess. Add steaks and cook 4 minutes on each side for a pink center. Season steaks with salt and pepper.