Place saffron in a small bowl with water; set aside.
Use a mortar and pestle to mash garlic and a pinch of salt to a fine paste. Add mustard and incorporate. Add egg yolks and mix well. Add saffron-water mixture and stir to incorporate. Slowly add half the oil, drop by drop at first, and then in a slow steady stream, while mixing with the pestle. (Thin with about a tablespoon of warm water if mixture is too thick.) Continue incorporating oil until all of it is absorbed. Thin with additional water if needed, and season with salt to taste.
The egg yolks in this recipe are not cooked. It should not be prepared for pregnant women, babies, young children, the elderly, or anyone whose health is compromised.