Preheat the oven to 425 degrees F. Line a baking sheet with parchment paper. Combine the beef, egg, cheese, breadcrumbs, onion, garlic, 2 teaspoons mustard, the Worcestershire sauce, paprika, 1 teaspoon salt and a few grinds of pepper in a medium bowl; mix with your hands until combined. Form into 1 1/2-inch meatballs (about 24) and arrange on the prepared baking sheet.
Mix the ketchup, brown sugar and remaining 1 teaspoon mustard in a small bowl and brush on the meatballs. Bake until lightly browned and cooked through, about 15 minutes. Transfer to a platter and top with scallions.
Photograph by Ryan Liebe