Pre-heat grill to medium. Grill tomatillos, peppers, onion and garlic until lightly charred and softened. When cooled, chop vegetables. Transfer to a bowl, add lime juice and cilantro; season to taste with salt and pepper.
Rub steak with olive oil and cumin; season liberally with salt and pepper. Grill for 5 minutes per side for medium-rare. When steak is done, transfer to plate, top with butter, and rest for 5 minutes. Slice thinly on a diagonal, drizzle with juices, and serve with salsa.