Seasoned with steak sauce and mustard, these flavorful sliders go great with almost any classic burger topper. The mini size is ideal for backyard potlucks, when everyone's plates are overflowing with salads, chips and watermelon.
In a large bowl, combine onion, broth, parsley (if using), steak sauce, mustard and pepper. Add ground sirloin and chuck; lightly mix together until just combined (do not overmix). Divide meat mixture into 12 portions and shape into 1/2-inch-thick patties.
Place patties on the greased rack of a gas or charcoal grill directly over medium heat. Cover and grill for 8 to 10 minutes or until done (160°), turning once. Wrap buns in heavy foil and add to grill rack the last 5 minutes of grilling, turning once.
Or preheat broiler. Place meat patties on the rack of a broiler pan. Broil 3- to 4-inches from the heat for 8 to 10 minutes or until done (160 degrees F), turning once halfway through broiling.