Wrap the outside and bottom of a 1-quart bowl in a damp towel to secure it in place on the work surface. Combine egg and salt with a large balloon whisk, mixing until foamy and pale. Whisking constantly, add oils a few drops at a time, and then in a steady stream as the mixture emulsifies. Add lemon juice; blend briefly. Add additional salt and lemon juice to taste. Store, tightly covered, in the refrigerator for up to 7 days.