4 ounces unsalted butter (1 stick), cut into 4 pieces
For the potato and tomatoes:
2 large potatoes, cooked, peeled, and sliced lengthwise
1 tablespoon chives, chopped
1 teaspoon olive oil
1 garlic clove, crushed
4 plum tomatoes, peeled, seeded, and cut into chunks*
*To peel tomatoes, immerse in boiling water for 30 seconds, cool in cold water, and remove skin. Split in half with a teaspoon and scoop out the seeds.
For the marinade:
Combine all ingredients for the marinade in a deep dish and mix thoroughly. Completely immerse the trimmed steaks in the marinade and refrigerate for 8 hours. Pour the marinade into a pot and bring to a boil, skimming off foam that rises to the surface. Reduce the marinade by half. Remove from heat and strain. Whisk in butter, one piece at a time, until it is totally incorporated into the sauce
For the steaks and potatoes:
Rub the cooked potatoes with garlic, oil, salt, and freshly cracked black peppercorns. Grill on both sides until golden brown. Place the tomato chunks on top of the potato and sprinkle with chopped chives. Serve grilled chived potato and tomato on warmed plate.
Season the sirloin steaks with pepper and grill to desired doneness. Let rest for 5 minutes before serving. Lay the sirloin steaks to one side of the grilled potato and tomatoes, and pour sauce around the steak.