Preheat your grill, then turn to getting the arugula topping ready. Heat the olive oil in a large skillet on medium heat. Add the garlic, sauté for a minute or so until it starts to become golden.
Stir in the arugula, getting it well-coated with the oil and garlic. Add a bit of salt and red pepper flakes, then cover. Cook briefly, about 3-4 minutes until just wilted. Keep covered on very low heat while you cook the burgers.
Spray the grill with oil, then add the burgers. Grill on direct heat, covered, for 5 to 6 minutes, then flip and grill for 5 minutes more. Add the cheese if using, give it another minute or so to melt.
Remove burgers from grill. Place the burger on a bun, then top with a scoop of the garlicky arugula mixture, and top with a slice of tomato. Enjoy!