Serve with classic Belgian-style Pommes Frites. A food processor helps make homemade mayonnaise quickly, but a whisk works just as well.
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.
Raw eggs should not be used in food prepared for pregnant women, babies, young children, the elderly, or anyone whose health is compromised.