1. Combine ground beef, chopped onion, minced garlic, soy sauce, dried thyme, salt, and pepper in a large mixing bowl. Mix well until all ingredients are fully incorporated.

2. Divide the mixture into small portions and shape each portion into a ball. You should end up with around 20-25 meatballs.

3. Heat a large skillet over medium-high heat (180-200°C or 350-400°F) and add butter. Once melted, add the meatballs and cook until browned on all sides, about 5 minutes.

4. Add veal stock to the skillet and bring to a simmer. Reduce heat to medium-low and cook the meatballs until cooked through, about 5-7 minutes.

5. While the meatballs are cooking, add sliced mushrooms to the skillet and cook until they are tender and lightly browned, about 5 minutes.

6. Add 2 sprigs of fresh thyme to the skillet and sauté for 1 minute. Season the mushroom gravy with salt and pepper to taste.

7. Serve the meatballs with the mushroom gravy spooned over the top, accompanied by a side of warm and comforting rice.