How to Make It
Heat grill to medium-high. Form the beef into four ¾-inch-thick patties. Use your fingers to make a shallow well in the top of each patty. (This will prevent overplumping during cooking.) Season with a total of ½ teaspoon each salt and pepper.
Oil the grill and cook the patties with the wells facing up for 4 to 5 minutes. Flip and cook until an instant-read thermometer inserted in the center registers 155° F for medium, 4 to 5 minutes more, topping the burgers with the manchego during the last 3 minutes of cooking.
Meanwhile, split the bread lengthwise and brush the cut sides with the oil; grill until toasted, 1 to 2 minutes. Rub the cut sides of the bread with the garlic. Cut the bread into quarters and form sandwiches with the watercress, burgers, and red peppers.