In a large skillet over medium heat, cook the bacon until crisp, about 10 minutes, turning occasionally. Drain the bacon on a plate lined with a paper towel.
Prepare the grill for direct cooking over medium-high heat (400° to 500°F).
In a large bowl mix the ground chuck, salt, pepper, and garlic powder, and then gently form four patties of equal size, each about ¾ inch thick. With your thumb or the back of a spoon, make a shallow indentation about 1 inch wide in the center of each patty to prevent them from forming a dome as they cook. Refrigerate the patties until ready to grill.
Brush the cooking grates clean. Brush the onion slices on both sides with oil, and then grill over direct medium-high heat, with the lid closed, until charred and caramelized on both sides, 6 to 8 minutes, turning once or twice. At the same time, grill the patties over direct medium-high heat until cooked to medium doneness (160°F), 8 to 10 minutes, turning once. During the last 30 seconds to 1 minute of grilling time, toast the rolls, cut side down, over direct heat.
To assemble the burgers, spread the tops and bottoms of the rolls lightly with mustard and then top each with lettuce, cheese, a patty, a strip of bacon, and an onion slice. Serve warm.