Finely grate the zest of 1 medium lemon into a large bowl (about 1 tablespoon). Juice the lemon until you have 2 tablespoons and add to the bowl. Add 1/2 cup mayonnaise, 1/4 cup plain yogurt, 1 tablespoon Dijon mustard, 1/2 teaspoon kosher salt, and 1/4 teaspoon black pepper, and stir to combine.