In a large mixing bowl, combine ground beef, breadcrumbs, Parmesan cheese, chopped onion, milk, egg, minced garlic, dried thyme, salt, and pepper. Use your hands to mix until just combined, avoiding overmixing to ensure tenderness.

Use your hands to form 1 to 1.5-inch meatballs and place them on a plate. Aim for uniformity in size for even cooking.

In a large skillet, heat a tablespoon of olive oil over medium heat. Add the meatballs in batches, ensuring there's space between each so they can brown evenly. Sear for 2–3 minutes on each side until they're golden brown.

In the same skillet, combine the French onion soup, beef broth, and Worcestershire sauce. Stir well, scraping the bottom of the pan. Allow this mixture to simmer for about 2 minutes, then pour it over the browned meatballs in the crock pot.

Cover the crock pot and cook on low for 4–5 hours. The meatballs should be tender and absorb the flavors of the French onion sauce.

Sprinkle shredded Swiss and mozzarella cheese over the top of the meatballs. Cover the crock pot again and let it sit for about 5 minutes, allowing the cheese to melt beautifully.

Carefully serve the meatballs warm, drizzling them with the rich French onion sauce from the crock pot.