Put the beef, onion, garlic, mustard, breadcrumbs, parsley, salt, pepper, paprika, Worcester sauce, grated Gouda cheese, egg, and thyme in a large bowl. Knead for 2-3 minutes until well combined.
Shape the mixture into meatballs, each weighing approximately 120 g.
Heat the butter and oil in a large pan over medium-high heat. Once the pan is hot, carefully add the meatballs. Reduce heat to medium, cover the pan, and fry for 7 minutes. Flip each meatball and fry for another 7 minutes until they reach an internal temperature of 71°C (160°F).