||In a skillet heat olive oil over medium-high heat, add onion slices and saute 5-6 minutes, stirring often, until softened and golden brown. Stir in BBQ spice rub and saute another minute. Add balsamic vinegar and 1/4 cup water and bring to a simmer.
||Stir in sausage slices and simmer 2 minutes to heat through and thicken slightly. Add more water, if desired, to make saucier and warm through.
||Ladle 1 cup of the sausage-onion mixture over each slice of bread and sprinkle with 1 tablespoon cheese, if desired. Place under a broiler to melt cheese before serving.