Transform your basic burger with this herby, garlicky sauce -- a traditional Argentinian condiment for beef.
Heat grill or grill pan to medium-high. Clean and lightly oil hot grill. Meanwhile, in a food processor, combine oil, herbs, vinegar, garlic, and 1 teaspoon salt. Pulse until finely chopped.
Season sliced onions with salt. Grill, turning frequently, until charred and tender, 10 to 14 minutes. Brush with chimichurri sauce and cook 1 minute more, flipping once. Transfer onions to a plate. Season patties with salt. Grill 3 1/2 minutes per side for medium-rare. Serve burgers on buns with onions, tomato, and arugula; top with chimichurri sauce.