Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.
In a mixing bowl, combine the ground beef, breadcrumbs, Parmesan cheese, chopped onion, minced garlic, egg, salt, black pepper, and dried thyme. Mix until well combined.
Form the mixture into meatballs, about 1.5 inches in diameter, and place them on the prepared baking sheet.
Bake the meatballs in the preheated oven for about 20 minutes, or until they are browned and cooked through (internal temperature should reach 160°F/70°C).
While the meatballs are baking, heat the French onion soup in a large skillet over medium heat until simmering.
Once the meatballs are done, carefully transfer them to the skillet with the soup. Simmer for an additional 5 minutes to allow the flavors to meld.
Sprinkle the shredded mozzarella cheese over the meatballs and cover the skillet. Allow it to cook for another 3-5 minutes, until the cheese is melted and bubbly.
Garnish with fresh parsley if desired before serving.