1 small white onion, chopped coarsely, divided
1 garlic clove, peeled and roughly chopped
1 tablespoon fresh lime juice
1/2 teaspoon cumin
1/4 teaspoon cayenne
1/2 teaspoon salt
1 pound beef skirt steak or flank steak,
    trimmed of surface fat

In a food processor or blender, combine one-quarter of the onion, the garlic, lime juice, cumin, cayenne and salt. Process to a smooth paste. Place the skirt steak in a non-aluminum baking dish. Using a spoon, smear the marinade over both sides of the skirt steak. Cover and refrigerate for at least 1 hour or up to 8 hours (NO MORE than 8 hours).

Remove the steak from the marinade. Oil the steak well on both sides and lay it over the hottest part of the grill. Grill, turning once, until richly browned and done to your liking, about 1 1/2  to 2 minutes per side for medium-rare.