In a large mixing bowl, combine ground beef, breadcrumbs, Parmesan cheese, egg, minced garlic, dried oregano, salt, and pepper. Mix well until all ingredients are incorporated, then shape the mixture into 1.5-inch meatballs. Place the formed meatballs on a parchment-lined baking sheet, ensuring they have some space between each one for even cooking.

Preheat your oven to 400°F (200°C). Once preheated, bake the meatballs for 20 to 25 minutes, or until they are browned and cooked through with an internal temperature of 160°F (70°C). After baking, simmer the meatballs in marinara sauce on the stovetop for an additional 10 minutes to infuse them with more flavor.

While the meatballs bake, heat a large pot over medium heat and add butter. Once melted, add the sliced onions and stir well. Cook the onions for about 30 minutes, stirring frequently, until they are deeply caramelized and golden brown, enhancing the flavor base for your French onion soup.

After the onions are caramelized, add dry white wine to the pot. Scrape any bits from the bottom of the pot to deglaze it and add depth of flavor. Allow the wine to simmer for about 5 minutes, then pour in the beef broth, thyme, and bay leaf. Bring the soup to a gentle simmer and cook for 20 minutes, allowing the flavors to meld beautifully.

Once both the meatballs and the French onion soup are ready, it's time to serve! Place meatballs alongside a generous bowl of soup. Top the soup with toasted baguette slices and a sprinkle of melted cheese.