Toast the cumin seeds in a small skillet over medium heat until they begin to brown lightly, and the smell wafts up. Let cool, then mince or grind in a mortar and pestle.
Whisk the yolks until thick, then add the lemon juice and garlic into a small bowl or a food processor fitted with a steel blade.
Painstakingly slowly, add first the grapeseed oil, then the olive oil, drop by drop to the egg yolk solution, whisking all the time. If you are using a food processor, dribble the oil through the tiny hole in the top. The mayonnaise will emulsify, thickening to a nice creamy texture.
Whisk in the cumin seeds, cover and chill for at least 2 hours.