Saffron Mayonnaise

Process egg, mustard, and 1/4 teaspoon salt in a food processor until pale and foamy, about 1 1/2 minutes. Combine oils; with machine running, add oil, drop by drop, through feed tube until mixture begins to thicken (about 1/4 cup oil; do not stop the machine). Add remaining oil in a slow, steady stream, processing until incorporated. Gradually add 1 tablespoon lemon juice.

Grind saffron with a mortar and pestle. Transfer to a bowl. Add 1 teaspoon lemon juice; let stand until liquid turns yellow, 5 minutes. Strain; discard threads.

Put mayonnaise into a small bowl; drizzle in saffron liquid, stirring. Season with salt, if desired. Cover; refrigerate until ready to serve, or overnight.