Grill tomatillos and jalapeño over medium-high until charred, 12 to 15 minutes. Chop jalapeño. In bowl, mash tomatillos. Stir in jalapeño, avocado, cilantro, garlic and lime juice; season. Brush steak with oil; season. Grill over high, 4 minutes per side for medium-rare. Let rest 10 minutes; slice. Serve with salsa.