Warm a medium fry pan over medium-high heat. Ladle 1 oz. butter into pan with two slices of bread. Flip bread over when browned and top each with a cheese slice.
On half of the sandwich, place 2 Tbsps. barbecue beef, 1/4 tsp. jalapeÒo pepper and onion slice. Place second slice of toasted bread on top and flip sandwich one more time to continue melting cheese.
Keep prepared grilled cheese warm in the oven while cooking the other sandwiches.