Finish caramelizing and build the broth
Carefully remove the pot from the oven and place it back on the stovetop over medium heat. If the onions are not yet golden brown, cook them for another 5 to 10 minutes, stirring frequently and scraping up any browned bits. Turn off the heat and pour in the wine. Turn the heat back to medium and cook for about 5 minutes so the wine reduces. Add the chicken broth, beef broth, Worcestershire sauce, garlic, salt, and pepper. Bring to a simmer, then reduce the heat to low and let the soup simmer gently for 30 to 45 minutes.
Tip: Scrape the bottom and sides of the pot as you stir to dissolve all the browned bits into the broth for maximum flavor.
Cleanup: If any broth spills while stirring, wipe it up right away to keep the stovetop clean and prevent burnt spots.