1. Preheat the oven to 375°F (190°C).

2. In a large bowl combine ground meat, egg, milk, salt, pepper, and chopped onion. Mix well with your hands until just combined.

3. Add dried thyme and mix again until the herbs are evenly distributed.

4. Use your hands to shape the mixture into small meatballs, about 1 ½ inches (3.8 cm) in diameter. Place them on a baking sheet lined with parchment paper.

5. Dot the top of each meatball with butter.

6. Bake the meatballs in the preheated oven for 12-15 minutes, or until cooked through.

7. While the meatballs are baking, prepare the mustard sauce. In a small bowl mix together 1 tablespoon of butter and 1 tablespoon of all-purpose flour. Cook over low heat stirring constantly until the mixture forms a smooth paste.

8. Gradually add ½ cup (120 ml) of milk, whisking continuously to prevent lumps. Bring the mixture to a simmer and cook for 2-3 minutes, or until it thickens.

9. Stir in 1 tablespoon of mustard and season with salt and pepper to taste.

10. Serve the meatballs hot, coated in the mustard sauce and garnished with fresh thyme.