1 large egg at room temperature
1 tablespoon Dijon mustard
1 tablespoon red or white wine vinegar
1/4 teaspoon kosher salt, or more to taste
1 cup (240 ml) neutral flavored oil, grapeseed, safflower or canola are best
Add egg to a quart jar process for 20 seconds using a handheld blender. Add the mustard, vinegar, and salt. Process for another 20 seconds.
Begin to slowly add the oil in tiny drops until about a quarter of the oil has been added (this is critical for proper emulsification).
When you notice that the mixture is beginning to thicken and emulsify, you can be a little less strict. With the blender on, continue to add it slowly, but increase to a very thin stream instead of drops of oil.
Taste mayonnaise for seasoning then add salt, lemon juice or extra vinegar to taste.
Note, if the mayo seems too thin, slowly stream in more oil with the processor running until thick.
Source: https://www.inspiredtaste.net/25943/homemade-mayonnaise-recipe/
