Place the aquafaba and cream of tartar in a blender and blend on low speed then slowly increase the speed to medium, until the liquid becomes foamy white, about 2 minutes. Add the mustard, onion powder, garlic, salt, pepper, and lemon juice and blend until smooth. With the motor running, pour the olive oil and blend until thick and creamy. Taste for seasoning. Dressing can be stored in the refrigerator for 3 to 4 days.
Notes
Extra chickpea liquid can be frozen for another use.