Egg Salad Tartines With Mixed Herbs

Combine egg yolk, mustard, and a pinch 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. 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.