Put the egg yolks, mustard, and lemon juice in a large bowl and begin mixing at high speed with an immersion blender or the whisk attachment of a stand mixer (or a handheld electric mixer). Continue to mix as you drizzle in the oil as slowly as possible. When the mixture has emulsified and all the oil is incorporated, add the miso and continue blending or mixing just until combined.
Put 1 inch of water in a large pot and add a pinch of salt. Put a steamer over the water, cover, and bring to a boil over high heat. Add the asparagus, cover again, and cook until the spears are just tender enough that you can pierce them easily with a knife, 7 to 10 minutes.
Rinse the asparagus with cool water, then drain thoroughly. Serve the asparagus dipped in the mayonnaise.
