Trim and chop the scapes. Blanch 1 minute in boiling water to mellow them, or leave raw for sharper bite.
Blend the egg yolk, scapes, and a big pinch of salt into a paste (if not blending finely dice the scape before whisking).
With the motor running (or whisking hard), drizzle the oil in a thin stream until thick and glossy. Taste and adjust salt.
Cut the veg into dippable pieces. Blanch asparagus or snap peas 60–90 seconds if you want them tender. Arrange and serve.