Beat together the egg, mustard, salt, and juice until combined.
Slowly pour in the oil, beating vigorously until thick and smooth.
Made a few weeks back using safflower oil. The flavor is very nice, and the texture pretty firm, though it does tend to be a bit loose for the purposes of tuna salad.
Made with 1 Tbsp. apple cider vinegar instead of lemon juice; used safflower oil again. I reduced the amount of vinegar used because I assumed vinegar would be more acidic than the lemon juice, but I think I should have used the full amount. The final flavor is quite flat - I may end up blending in the rest of the vinegar later. The consistency seems looser this time, but still holds its form on a dish for a while.