Start by placing the egg yolk in a jar. Make sure the jar is clean and dry.
Add the horseradish apple cider vinegar and salt to the jar with the egg yolk.
Whisk the mixture vigorously with a whisk or use an electric hand mixer on low speed until everything is well combined and slightly frothy.
Now, this is the most important part: very slowly, start adding the vegetable oil. Drizzle it in a super thin, steady stream while continuously whisking or mixing. If you add the oil too fast, the mayonnaise might not emulsify properly and could separate.
Keep whisking and adding the oil until all of it is incorporated and the mixture has thickened into a creamy, pale yellow mayonnaise.
Enjoy this super delicious Mayonnaise. It's simple, tasty, and brings back those good old memories!