8 small slider rolls, toasted or warmed in the oven
Salt and pepper, to taste
Heat 2 tablespoons of olive oil in large sauté pan until almost smoking. Add the chopped onions, salt, sugar, and pepper and sauté over high heat until the onions start to color. Reduce heat to low. Cook onions over low heat, stirring occasionally, until onions are very soft and caramelized—about 30 minutes. Set aside until needed.
Shape the ground beef portions into patties, slightly larger than the diameter of the slider rolls—about 3 inches across.
Season patties lightly with salt and pepper.
Heat 1 Tablespoon olive oil in large sauté pan until almost smoking. Add the patties, in batches if necessary, and cook to an internal temperature of 160F, or about 6 minutes.
Place cooked slider on bottom half of roll. Spread top of roll with 2—3 teaspoons of caramelized onion.
Spread about 1 Tablespoon of softened Boursin cheese on the patty. Close the sandwich and toothpick if desired.