Combine beer, oil, onion, cilantro, onion, cilantro, salt, and pepper in a blender; blend until cilantro marinade is smooth.
Place steak in a large resealable plastic bag and pour cilantro marinade on top. Seal bag and shake to fully cover meat with the marinade. Refrigerate at least 8 hours or overnight.
Preheat an outdoor grill for medium-high heat and lightly oil the grate with avocado oil. Grill the meat, turning over once, about 3 minutes per side for medium rare.
