Step 1

Combine egg yolk, mustard, and a pinch of salt in a medium bowl. Anchor bowl by setting it inside a towel-lined saucepan, which will afford you 2 free hands—1 for pouring and 1 for whisking. Whisk until very smooth.

Step 2

Combine olive and vegetable oils in a measuring glass and pour into yolk mixture, drop by drop at first, then in a slow steady stream as mixture stiffens, until all the oil has been added. Whisk in vinegar and hot sauce; season with salt and pepper. Thin mayo with water if needed, adding a teaspoonful at a time until just loose enough to fall off a spoon.

Step 3

Gently lower eggs into a large saucepan of boiling salted water, and cook, maintaining an active simmer, 9 minutes. Immediately transfer eggs with a slotted spoon to a bowl of ice water and stir until cool enough to handle, about 1 minute. Drain, pat dry, and peel eggs. Tear eggs into large pieces and add to mayonnaise. Gently fold just to coat; season with salt and pepper. Spoon over bread.

Step 4

Toss scallions, basil, dill, chives, and lemon zest in a small bowl. Add lemon juice, drizzle with oil, and season with salt. Toss to coat. Pile herbs on top of egg.