Whisk the garlic, lemon juice or vinegar, mustard, rice vinegar, a big pinch of salt, and several grinds of pepper in a medium bowl or a 1-quart liquid measuring cup.
Slowly whisk in the oil (I frequently pull out my hand mixer fitted with the whisk attachment), first in droplets, then in a steady stream to make a thick vinaigrette.
Pour into a jar and refrigerate. (Can be refrigerated for up to 1 month.)
Makes 1-½ cups