Combine egg yolks and mustard in a food processor. Blend on high for 30 seconds - 1 minute.
Slowly add the oil to the mixture. Start by adding half a tablespoon and check for a creamy texture resembling mayonnaise. Make sure it's not watery before adding more oil. Be patient and ensure the ingredients blend well. (Tip: Increase the speed of adding oil slightly once you see a creamy texture.)
Once the mixture transforms into creamy mayonnaise, add salt, pepper, and lemon juice.
Gently mix for a thicker texture or fold the ingredients together.
Notes
Use room-temperature eggs: Cold eggs are one of the most common causes of a broken (separated) mayo; let your eggs sit out before starting.
Add the oil slowly: Stream the oil in gradually rather than pouring it all at once; adding it too fast is the other main reason mayo breaks.
Blend from the bottom: Keep your blender or immersion blender at the bottom of the container as it emulsifies, then slowly raise it once it starts to thicken.
Oil choice matters: Stick with a neutral oil like canola, vegetable, or avocado oil. Extra virgin olive oil is too strong and will overpower the flavor.
If it breaks: Don't toss it, start with a fresh egg yolk in a clean bowl and slowly whisk in the broken mixture to bring it back together.
Too thick? Thin it out with a splash of water or a little extra lemon juice.
Customize it: Once made, stir in fresh herbs, garlic, or spices to flavor it however you like.
Storage: Keeps up to 1 week in the fridge in an airtight container; always use a clean utensil to scoop it out.