1. Prepare Chunky Ketchup.
2. Trim fat from steak. Score steak on both sides by making shallow cuts at 1-inch intervals in a diamond pattern. Place steak in plastic bag set in shallow dish.
3. Add 1 cup of the ketchup to plastic bag with meat. Seal; marinate overnight in refrigerator 6 to 24 hours, turning bag occasionally. Drain steak; discard ketchup.
4. For charcoal grill, arrange medium-hot coals around drip pan. Test for medium heat above pan. Place steak on grill rack over drip pan. Cover; grill 23 to 28 minutes for medium (160 degrees F), turning halfway through grilling. For gas grill: Preheat grill. Reduce heat to medium. Adjust for indirect cooking. Grill as above. Cover and keep warm.
5. Heat 1 cup of remaining Chunky Ketchup to serve with steak (or serve cold). Tightly cover and refrigerate remaining ketchup for another use. Thinly slice steak diagonally across the grain. Serve steak with Buttermilk Mashed Potatoes.