Make the Creamy Pickled Beet Salad: Combine all ingredients in a small bowl and stir to combine.
Assemble the Sandwiches: Transfer bread slices to a clean work surface. Butter the bread. Top with a handful of arugula, a generous spoonful of Creamy Pickled Beet Salad. Artfully arrange meatball slices on top. Garnish with herbs and serve.