1 cup neutral oil (240 mL), such as canola or vegetable
1 tablespoon lemon juice
1 teaspoon kosher salt
In a food processor, combine the egg, mustard powder, and vinegar. Puree for 1 minute, until pale yellow.
With the processor running, very slowly start to add the oil, 1 tablespoon at a time. Once you’ve added half of the oil, add the rest in a slow, steady stream. The mayonnaise should begin to thicken to the consistency of prepared mayo, or a bit looser. Add the lemon juice and salt and pulse to incorporate.
Transfer to a clean, airtight container and store in the refrigerator. The mayonnaise should keep for up to 1 week.