Slice the chilled radishes into 1/8-inch rounds using a sharp knife or mandoline for uniform thickness. The thin slices are important because they allow the butter and salt to coat evenly and make each bite delicate and refined. Arrange the slices on a serving plate in slightly overlapping rows.

Spread small dollops of room-temperature butter across the radish slices, using about 1/2 tablespoon per 3 radishes so they're generously but not heavily coated. I find that letting the butter sit at room temperature beforehand makes it spreadable without tearing the delicate radish slices. Sprinkle the Maldon sea salt flakes evenly over the buttered radishes, then garnish with the fresh chives and lemon zest just before serving.