Combine the egg yolks, mustard and lemon juice in the bowl of a food processor, blender, or stand mixer.

Combine the bacon fat and oil in a liquid measuring cup.

With the machine running, gradually add the mixture, a few drops at a time, to the egg mixture.

Gradually increase to a thin stream to form a fully emulsified mayonnaise.

Season to taste with salt and pepper.

Refrigerate until ready to use.

Because of the bacon fat, the mayonnaise may harden more than usual when it is chilled.

It can still be spread, or you can leave it at room temperature for 30 minutes to soften.