6 ounces red radishes, very thinly sliced on a mandoline
In a small bowl, blend the softened butter with the lemon zest and lemon juice and season with salt.
Spread the bread slices with the lemon butter. Using a 1 1/2-inch round biscuit cutter, cut 16 rounds from 4 slices. Cut the remaining slices into 2 1/2-by-1-inch rectangles. Top with the parsley leaves and radish slices. Serve at once, or cover the sandwiches with plastic wrap and let stand at room temperature for up to 2 hours.