Gently combine meat, garlic powder and 1/4 cup dressing just until blended; shape into 8 (1/2-inch-thick) patties. Use your thumb to indent each patty to help the burgers maintain their shape. Grill 5 to 6 minutes on each side or until done (160ºF).

While the burgers are cooking, in a large bowl, toss together the coleslaw mix, pineapple, onions (and cilantro, if desired). In a small bowl, whisk together the remaining dressing and the sriracha sauce (and lime juice, if desired). Add the dressing to the coleslaw mix and toss to combine. Season to taste.

During the last few minutes of cooking, brush the buns with butter or extra-virgin olive oil and place them on the grill, cut-side-down on. Grill the buns for about 1 minute or until toasted. Remove the buns and burgers from heat. Place the burgers in the buns, top with the slaw, and serve immediately.