With a mortar and pestle, crush the garlic and the egg yolk. Slowly pour in the vegetable oil (drop by drop) and continue mixing with the sauce. Continue until it starts to bind and you have at least 2 tablespoons of thick mayonaise.
Slowly add a little more, just a spoonful of vegetable oil at a time until all is used up. Then, season with mustard, lemon, vinegar, and mustard seeds. Season to taste with salt and olive oil for flavour