Add the egg, lemon juice, vinegar, mustard, salt and pepper to the jug of your high-powered blender. Blend on medium, until combined.
Slowly drizzle in warmed bacon grease, a little bit at a time so that the mixture has time to thicken. Drizzling in should take about 1 minute total. Slow and steady wins the race!
Continue to run mixture for another 1-2 minutes, allowing it to thicken.
Transfer to a wide-mouth mason jar, cover and refrigerate. Should keep in the fridge for up to 5 days.
serving = 2 tablespoons
Notes
Mustard: I liked the mayonnaise without mustard but it's a preference thing. Eggs: if you want to make this recipe as high-fat, keto as possible, use 2 egg yolks in place of the 1 egg. I liked it with 1 egg because the egg whites make the mayonnaise extra fluffy. Consistency: right out of the blender, the mayonnaise will be runny. If left at room temperature, it will harden slightly. When refrigerated, it will harden to the consistency of butter.
Recipe by Healthful Pursuit at https://www.healthfulpursuit.com/2014/08/bacon-grease-can-be-your-friend/