Preheat the grill. Mix together the sour cream, horseradish and lemon juice. Season with salt and pepper. Brush the bread slice with the olive oil. Place the bread on a grill and grill for 2 minutes per side, until the bread becomes crunchy. Remove the bread from the grill and cut each slice in half. On 12 of the bread halves, spread some of the horseradish sauce. Top the sauce with 2 ounces of roast beef. Top the beef with 1 leaf of Romaine lettuce. Place the remaining bread halves on top of the Romaine.