How to Make It
Place vinegar, shallot rings, and ¼ teaspoon of the salt in a bowl; let stand at least 10 minutes. Drain shallot rings; reserve pickling liquid for another use.
Bring a large pot of water to a boil over high. Gently place eggs in boiling water, and boil 6 minutes. Transfer eggs to a bowl of ice water to cool; peel.
Stir together butter, chives, several grinds of pepper, and remaining ⅛ teaspoon salt in a small bowl until combined.
Divide chive butter among bread slices, spreading to edges; top with radishes. Tear eggs open with your fingers, and place on top of radishes. Top evenly with pickled shallot rings, and sprinkle with flaky salt and several grinds pepper.