We're using yellow mustard, but you can substitute with Dijon mustard for a sharper flavor or Creole mustard for a spicy kick.
Use freshly squeezed lemon juice here as opposed to the bottled kind. The fresh flavor makes a huge difference. You will need 1 1/4 lemons (so grab two) to yield 1/4 cup of juice.
Feel free to add 1-2 teaspoons of capers to the mix before blitzing.