Preheat your oven to 425°F. Line a large baking sheet with parchment paper.
Mix all of the meatball ingredients until combined. Form your medium sized meatballs; I produced 16 meatballs. Bake them for 20-25 minutes.
French Onion Sauce:
Melt your butter with the oil in a large deep pan or wide dutch oven. Add the onions and cook on LOW HEAT until they soften and caramelize, should take 30-45 minutes. Stir occasionally.
Stir in the garlic for 30 seconds. Pour in the beef broth, thyme and season with salt and pepper to taste. Bring to a boil and then lower heat to a simmer. Cook for about 10-15 minutes, allowing it to thicken.
Add the cooked meatballs to the sauce pan and sprinkle the cheese on top. Cover the pan for the cheese to melt.
To serve: Garnish with fresh parsley. Add meatballs and onions on top of your bread slices.
Notes
You can serve with the sourdough bread slices or mashed potatoes, rice or french fries.