Begin by melting 4 tablespoons of unsalted butter in a large skillet over medium heat. Once melted and bubbling, whisk in ¼ cup of all-purpose flour until smooth and golden for about 2-3 minutes.

Slowly pour in 2 cups of beef broth while whisking constantly to avoid lumps. Bring to a simmer, cooking for 5-7 minutes until thickened. Season with 1 teaspoon of kosher salt, ½ teaspoon of ground black pepper, and a dash of ground nutmeg. Stir in 1 cup of ground beef, ½ cup of chopped onion, and ¼ cup of parsley.

Cook the filling until the ground beef is mostly browned, about 5-7 minutes. Transfer to a container and cool for 20 minutes at room temperature, then refrigerate for at least 3 hours or overnight.

Set up your breading station with ½ cup of flour, 2 beaten eggs, and 1 cup of breadcrumbs in separate bowls.

Shape the cooled filling into small balls, about 1 inch in diameter. Roll in flour, dip in eggs, and coat with breadcrumbs.

Heat vegetable oil in a deep fryer or Dutch oven to 375°F (190°C). Fry meatballs for about 2.5 minutes or until golden brown. Drain on paper towels.

Serve warm with an array of mustards for dipping.