- Whisk spring onion, garlic, coffee, vinegar, mustard, brown sugar, oil, and 1 tsp. pepper in a medium bowl. Pour half of marinade into a resealable plastic bag. Add steak, seal bag, and turn to coat. Let sit at room temperature 1 hour.
- Cover remaining marinade and set aside.
- Prepare grill for medium-high heat. Remove steak from marinade; discard used marinade. Season steak with salt and pepper and grill, turning and basting often with reserved marinade, 8–10 minutes for medium-rare. Let rest 10 minutes before slicing.
- Do Ahead: Steak can be marinated 1 day ahead. Chill, turning occasionally. Chill reserved marinade. Bring steak to room temperature before grilling.