1. Preheat your oven to 375°F (190°C).

2. In a large mixing bowl, combine 450g (1 pound) ground beef, 1 medium yellow onion finely chopped, 1 egg, 1 teaspoon salt, and 1/2 teaspoon pepper. Mix well with your hands until just combined.

3. Divide the mixture into small portions, roughly the size of a golf ball, and shape each into a meatball. Place the meatballs on a baking sheet lined with parchment paper, leaving a little space between each one.

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

5. While the meatballs are baking, prepare the mushroom gravy. In a large saucepan, melt 2 tablespoons (30g) of butter over medium heat. Add 250g (1 cup) sliced mushrooms and cook until they release their liquid and start to brown.

6. Add 250ml (1 cup) veal stock, 1 tablespoon soy sauce, and 1 tablespoon light roux to the saucepan. Whisk until smooth. Bring the mixture to a simmer and cook for about 5 minutes, or until thickened.

7. Add 60ml (1/4 cup) heavy cream to the saucepan and whisk until combined. Bring the mixture to a simmer and cook for an additional 2-3 minutes, or until heated through.

8. Serve the meatballs with the mushroom gravy spooned over the top and accompanied by mashed potatoes on the side.