In a small bowl, whisk together the room temperature egg yolk, lemon juice, vinegar, mustard, and salt. Whisk until frothy.
Whisking briskly and constantly, begin drizzling in the oil a few drops at a time. Once the mixture starts to emulsify and thicken, add the oil in a thin, slow and steady stream. Continue whisking and slowly adding the oil for 3 to 5 minutes, until all of it is incorporated and the mayonnaise has thickened.
Store in a glass container in the refrigerator for up to 4 days. You can use immediately, but it thickens with refrigeration.
While you can easily make keto mayonnaise by hand with a whisk or hand mixer, a stand mixer is useful so your hands are free to easily stream in the oil.