Place the egg, lemon juice, and mustard powder in a food processor and pulse. Then in a bowl mix the avocado and olive oil. Then slowly, drop by drop for a few teaspoon, add the oil to the processor, while mixing. This part is important, because you need it to emulsify! After a few teaspoons, then pour it into the drip section of your food processor or continually add the oil slowly. This should make a mayonnaise.