Heat a grill to high (450° to 550°). Using a sharp knife, score steak on a diagonal across the grain, making parallel lines about 1 in. apart. Turn steak and repeat to create a diamond pattern. Flip steak over and repeat scoring on the other side.
Combine 1 tsp. salt and the spices, then sprinkle all over steak. Drizzle meat with butter and lemon juice and rub flavorings into slits.
Grill steak, turning once, 8 minutes total for medium. Let rest. Mix onion and peppers with oil, garlic, and remaining 1/2 tsp. salt. Grill, turning often, until softened, 5 minutes. Cut vegetables into strips; serve with steak.