In a small bowl, whisk the oil, garlic, fennel thyme, 1 tablespoon of salt, 2 teaspoons of pepper. Rub the paste over the beef and bring to room temperature.

Light a grill. Grill the roast over moderate heat, turning often until charred and registered at 118 degrees on a thermometer. 35-40 minutes. Tent with foil on a carving board and let cool for 30 minutes. Wrap roast tightly and let cool completely. Refrigerate overnight.

In a bowl, whisk the buttermilk, mayonnaise and the 2 tablespoons of horseradish. Season with salt and pepper. Carve the roast into 1/4 inch thick slices and serve cold with the buttermilk sauce and more freshly grated horseradish.