Egg: 1 large (preferably room temperature)
Oil: 3/4 to 1 cup (light oils like sunflower, canola, or avocado are recommended) olive oil
Acid: 1 tablespoon (lemon juice or vinegar)
Mustard: 1 tablespoon (Dijon is commonly used)
Salt: to taste
Method
Combine Ingredients: In a tall, narrow jar, add the egg, mustard, acid, and salt. Pour the oil on top.
Blend: Using an immersion blender, place it at the bottom of the jar over the egg yolk. Blend on high without moving for about 10-15 seconds until it starts to thicken. Then, slowly lift the blender up and down to incorporate all the oil.
Taste and Adjust: After blending, taste the mayonnaise and adjust with more salt or acid if needed.