Heat a charcoal or gas grill until hot (you should barely be able to hold your hand 3 or 4 inches from the fire), or use a grill pan or medium-sized skillet on medium-high heat.
Bring the steak to room temperature, pat dry with paper towels, trim fat from steak, and sprinkle both sides with salt and pepper.
Meanwhile, husk, wash, and quarter the tomatillos. Put them in a food processor along with the garlic clove, shallot, cilantro, jalapeño pepper, lime juice, and 1/4 teaspoon each salt and freshly ground pepper. Cover and pulse until the mixture is puréed.
Grill steak until nicely browned, 3 to 4 minutes per side. Move the meat to a cooler part of the grill and cook another 5 to 7 minutes, turning once. Cover and let rest for 5 minutes before cutting across the grain into thin slices. Serve drizzled with salsa.