In a bowl, toss the radishes with the dill. In another bowl, stir together the butter, lemon zest and pepper. Spread the lemon butter on one side of each bread slice; cover with the radish mixture and sprinkle with salt. Using a 2 1/2- to 3 1/2-inch star-shaped cookie cutter, cut out the sandwiches. Refrigerate for 1 hour.