Combine ground beef, garlic powder, onion powder, sea salt, Italian seasoning, black pepper, egg, bread crumbs, Worcestershire sauce, and whole milk in a large bowl, mixing thoroughly by hand or using a stand mixer with a paddle attachment.

Shape the mixture into meatballs about the size of golf balls (approximately 2 tablespoons each), yielding around 24. Moisten hands with water to facilitate rolling.

In a large Dutch oven or heavy-bottomed pot, heat 2 tablespoons of olive oil over medium-high heat. Sear the meatballs on all sides without fully cooking them through. Work in batches to avoid overcrowding. Transfer browned meatballs to a plate.

Deglaze the pot with a splash of beef stock if needed. Add the remaining tablespoon of olive oil and sauté the onion, celery, and carrots until they begin to soften, about 5 minutes. Stir in the minced garlic and cook for another minute until aromatic.

Mix in tomato paste and thyme, ensuring even distribution. Return the meatballs to the pot and pour in the beef stock along with the beef bouillon. Increase heat to bring to a boil, then lower to a simmer.

Introduce the potatoes and green beans to the stew. Cover with a tight-fitting lid and simmer for 20 minutes, or until the vegetables are tender and meatballs reach an internal temperature of 160°F. To keep green beans crisp, add them 10 minutes after the potatoes.

Whisk together cornstarch and cold water in a small bowl until smooth. Stir the slurry into the stew and let it simmer on low until thickened, approximately 5 to 7 minutes.

Ladle into bowls, garnish with parsley, and serve hot alongside crusty bread or buttered egg noodles.