Preheat oven or grill to 350 degrees F. Line a baking sheet with parchment paper for easy clean up.
Melt butter in saute pan over medium heat. Cook onions until fragrant and slightly brown while stirring occasionally, about 3-5 minutes. Reduce heat to low and add garlic, cook for 2-3 minutes. Remove from heat and set aside to cool.
In large bowl, combine ground beef, egg, bread crumbs, milk, Worcestershire sauce, thyme, salt and cooked onions and garlic. Use hands to mix well, but try not to over work the meat. Shape into 1-2 inch sized balls on parchment lined baking sheet.
Bake uncovered in the preheated oven 20 minutes. While meatballs are cooking, combine ketchup with mustard and brown sugar in a bowl. Set aside.
After 20 minutes of cooking, dip each meatball in ketchup mixture, return to baking sheet, and continue baking for about 10 additional minutes. Serve warm with toothpicks for easy eating. Can be kept warm in slow cooker, if needed.