The BEST French Onion Soup recipe! Made with slowly caramelized yellow onions, a savory lightly herby broth, and each serving is finished with golden brown, rich cheese toasts. Talk about delicious!
Melt butter in a large pot* (about 6 quart) over medium heat.
Add onions and salt and saute tossing occasionally, for 45 to 50 minutes - but keep reading and proceed with sauteeing process as noted. For the first 15 minutes toss about every 3 to 4 minutes. Reduce burner temperature to medium-low if onions are browning too quickly or browning along bottom of pot.
From here you'll need to cook about 30 minutes longer, and start tossing the onions more frequently about every 2 minutes then even down to every minute. As you toss the onions throughout the entire sautéing and caramelization process use a wooden or bamboo spatula to scrape up light browned bits from bottom of pot.
Also as you notice any light browning on the bottom of the pot that doesn't scrape up scoot the onions from that area, add a splash of water (about 1 Tbsp) then let it steam and scrape with the spatula (it can be scrapped into the onion mixture). This will really keep onion soup from tasting burnt.
Once onions are caramelized add flour and saute 1 minute.
Pour in white wine and scape up golden brown bits from bottom, and stir well to smoothly incorporate flour. Add beef stock, chicken broth, thyme and bay leaves, salt and pepper to taste and bring to a simmer over medium-high heat.
Reduce heat to low, cover and simmer 15 to 20 minutes.
Meanwhile move oven rack about 6 inches from top element and preheat oven to 450 degrees. Rub garlic halves over one side of each baguette slice then align on a 18 by 13-inch rimmed baking sheet. Bake in preheated oven until slightly golden brown and crisp about 5 to 8 minutes.
Once soup is done, remove thyme and bay leaves.
Preheat the oven to broil. Ladle soup into broil proof bowls filling no more than about 2/3 full (I used 20 oz bowls and added 1 1/2 cups soup).
Working with up to 4 bowls at a time place them on rimmed baking sheet (same one used to toast bread) top each soup with two baguette slices.
Sprinkle with plenty of the gruyere and parmesan cheese then place about 6 inches from boiler (still set on baking sheet) and broil until cheese is nicely melted.
Serve right away garnished with parley if desired.
Notes
I recommend using a heavy pot with a light colored interior so you can see when there is excess browning on the bottom of the pot to deglaze. I prefer an enameled cast iron pot rather than stainless steel or non-stick, onions don't burn as easily.