In this take on chimichurri--a green herb sauce for grilled meats that originated in Argentina--we sub in softened butter for traditional oil.
Heat grill to high. Stir together butter, shallot, parsley, cilantro, and vinegar in a bowl; season generously with salt and pepper. Toss asparagus with 1 teaspoon oil; season with salt and pepper. Pat steak dry with paper towels, then rub with remaining 1 teaspoon oil; season with salt and pepper. Let stand 10 minutes.
Grill steak, flipping once, until charred in places and a thermometer inserted into thickest part registers 125 degrees for medium-rare, 8 to 10 minutes total. Transfer to a carving board and slather with some of chimichurri butter; let rest 10 minutes. Meanwhile, grill asparagus, turning a few times, until charred in places and crisp-tender, 6 to 8 minutes.
Thinly slice steak against the grain. Serve steak and asparagus, topped with remaining chimichurri butter.