In a medium skillet, heat olive oil over medium-high. Add onion, and cook until softened, 2 minutes. Season with salt and pepper. Add sugar and vinegar; toss to coat. Let mixture cool. In a small bowl, combine garlic and mayonnaise.
Heat a grill to medium-high. Clean and lightly oil hot grill. Divide ground beef into 8 balls. Press each lightly to form 3/4-inch-thick patties. Season each with salt and pepper. Grill 3 minutes per side for medium. Serve sliders on rolls with pickled onions and garlic mayonnaise.
Indoor method: In an oiled grill pan or cast-iron skillet, cook burgers over medium-high.