Homemade Mayonnaise for Pommes Frites

Combine oils in a glass measuring cup. Place egg, mustard, and salt in the bowl of a food processor. Blend until mixture is foamy and pale, about 1 1/2 minutes.

With the machine running, add the oil, drop by drop, through the feed tube, until the mixture starts to thicken (about 1/4 cup oil); do not stop the machine at this point or the mayonnaise may not come together. Add the remaining oil in a slow, steady stream. When all the oil has been incorporated, slowly add the lemon juice. The fresh mayonnaise can be kept, refrigerated in an airtight container, for up to 3 days.