Mix ground beef, breadcrumbs, and egg together.
Add French onion soup mix.
Season with salt and pepper.
Form into 1-inch balls.
Preheat oven to 375°F.
Place meatballs on baking sheet.
Bake for 20 minutes until golden brown.
Melt butter in large skillet over medium heat.
Add sliced onions.
Cook until caramelized, about 15 minutes. Stir frequently.
Pour beef broth into the onions.
Add Worcestershire sauce and thyme.
Bring to gentle simmer.
Add baked meatballs to the sauce.
Simmer for 10 minutes.
The sauce will thicken beautifully.