Mix beef and pork together in a large bowl to thoroughly combine.
Add the breadcrumbs, eggs, parsley, and onions. Mix thoroughly.
Roll into meatballs, about 0.5 cup each, and dredge in flour.
Heat butter in a large skillet over medium heat.
Add the meatballs and brown on all sides, about 8 to 10 minutes on each side.
Remove meatballs to a plate. Melt 2 tablespoons of butter to the skillet.
Add the chopped onions and reduce heat to medium-low. Cook until onions are caramelized.
Add the brown sugar and thyme, stir and cook for 1 minute.
Add the flour, stir to combine.
Add the syrup Liège {apple butter} and vinegar, stir.
Pour in the beef stock, and add the bay leaves. Bring to a boil, reduce heat and simmer for 25 minutes.
Add the meatballs back to the sauce and cook for 15 minutes to heat through.
Serve immediately with pomme frites {French fries} and Belgian beer or wine.
