In a medium bowl combine the butter ingredients, including ½ teaspoon salt and ½ teaspoon pepper, and mix well. Cover and refrigerate until needed.
Prepare the grill for indirect cooking over medium heat (180° to 230°C) and preheat a perforated grill pan for about 10 minutes.
Prepare the grill for cooking over medium heat (200°C) and preheat a perforated grill pan for about 10 minutes.
In a large bowl whisk the oil, mustard, ½ teaspoon salt, and ¼ teaspoon pepper. Reserve 1 tablespoon of this mixture in a small bowl. Cut each sweet potato lengthwise into quarters, and then cut each quarter into slices about 12 millimeters thick. Add to the large bowl with the oil-mustard mixture and turn to coat.
Spread the sweet potatoes in a single layer on the grill pan and grill over indirect medium heat, with the lid closed, until tender, 30 to 35 minutes, turning occasionally. Put the potatoes back into the large bowl and toss with the reserved tablespoon of the oil-mustard mixture. Tent with foil to keep warm.
Spread the sweet potatoes in a single layer on the grill pan and grill over medium heat, with the lid closed, until tender, 30 to 35 minutes, turning occasionally. Put the potatoes back into the large bowl and toss with the reserved tablespoon of the oil-mustard mixture. Tent with foil to keep warm.
Season the steaks evenly on both sides with salt and pepper. Allow to stand at room temperature for 15 to 30 minutes before grilling.
Change the method and increase the temperature of the grill for direct cooking over medium-high heat (200º to 260ºC). If you're using a charcoal grill, keep a small part of the charcoal grate clear of briquettes as a safety zone of indirect heat.
Grill the steaks over direct medium-high heat, with the lid closed, until cooked to your desired doneness, 6 to 8 minutes for medium rare, turning once or twice (if flare-ups occur, move the steaks temporarily over indirect heat). Remove from the grill, top with a spoonful of the horseradish butter, and let rest for 3 to 5 minutes. Serve warm with the Dijon sweet potatoes.
Grill the steaks over medium-high heat, with the lid closed, until cooked to your desired doneness, 6 to 8 minutes for medium rare, turning once or twice (if flare-ups occur, move the steaks temporarily over indirect heat). Remove from the grill, top with a spoonful of the horseradish butter, and let rest for 3 to 5 minutes. Serve warm with the Dijon sweet potatoes.
