Prepare an ice-water bath. Bring a large pot of water to a boil; season generously with kosher salt. Working in batches, cook green vegetables until just tender and bright green, 2 minutes for asparagus, 3 minutes for Romanesco and peas, and 1 minute for fiddleheads. Transfer to ice-water bath. Drain; pat thoroughly dry.

Heat 6 tablespoons butter in a small bowl set over a pot of barely simmering water until almost melted but not quite translucent, 1 to 2 minutes. Remove from heat; whisk in remaining 6 tablespoons butter, 1 tablespoon at a time, until mixture is opaque. Whisk in lemon zest.

Line two rimmed baking sheets with parchment. Dip half of each cooked and raw vegetable in butter, letting excess drip off; transfer to baking sheets. Sprinkle with pink salt. (If butter becomes too thick, place over simmering water to gently reheat.) Refrigerate until butter hardens, at least 20 minutes or, loosely covered, up to 2 hours; serve cold.