Mix cumin, coriander, chili powder, salt and 3/4 teaspoon pepper in a small bowl. Rub steak with the cut garlic. Rub the spice mix all over the steak.
Heat 2 teaspoons oil in a large heavy skillet, preferably cast iron, over medium-high heat. Add the steak and cook to desired doneness, 4 to 6 minutes per side for medium-rare. Transfer to a cutting board and let rest. dd remaining 1 teaspoon oil to the skillet. Add bell peppers and onion; cook, stirring often, until softened, about 4 minutes. Add minced garlic and brown sugar; cook, stirring often, for 1 minute. Add coffee, vinegar and any accumulated meat juices; cook for 3 minutes to intensify flavor. Season with pepper.
To serve, mound 1 cup watercress on each plate. Top with the sauteed peppers and onion. Slice the steak thinly across the grain and arrange on the vegetables. Pour the sauce from the pan over the steak. Serve immediately.