Preheat oven to 400 degrees. Cut the top off of the bulb of garlic and drizzle with a little bit of olive oil and a sprinkle of salt and pepper. Wrap in foil and place in the oven for 45 minutes. Now you have roasted garlic (YAY!). Squeeze garlic out of their casing and set aside.
Place the egg, 1/4 cup of olive oil, mustard powder, reserved garlic, and salt in a mixing bowl, blender, or food processor. Mix thoroughly. While the food processor or blender is running, slowly drizzle in the remaining cup of olive oil. After you’ve added all the oil and the mixture has emulsified, add lemon juice to taste, stirring gently with a spoon to incorporate.