1. Heat grill to medium for 10 minutes, then clean and oil grates. Meanwhile, divide beef into eight equal portions, then form each into a thin, 4"-wide patty. Stack four cheese squares in center of each of four patties, then top with remaining patties and firmly pinch edges together to seal in cheese. Season all over with salt and pepper. Cover and refrigerate until ready to cook.
2. Brush onion slices with 1 tablespoon oil and season with salt and pepper. Grill, covered, until well-marked, 5 to 7 minutes. Flip and continue to grill, covered, until tender, about 5 more minutes. Transfer to a cutting board and coarsely chop, then place in a small bowl and stir in remaining 1 tablespoon oil, the vinegar, sugar, and 1/4 teaspoon each salt and pepper. Set aside.
3. Increase heat to medium-high and grill patties, covered, until well-marked and cooked through, about 6 minutes per side.
4. Divide patties among bun bottoms, then top with relish, lettuce, tomato, and bun tops. Serve immediately.
Source: Hannaford fresh Magazine, May-June 2021