1) Preheart oven to 350°F
2) Sauté the onions in a pan with butter and the maple syrup for 15 minutes over medium heart, stirring occasionally. Continue cooking until the onions are caramelized (about 15 minutes more). Add the garlic and stir for a few seconds.
3) Deglaze by adding the white wine vinegar.
4) Add the beef broth, chicken broth, thyme laurel leaves and flour. Mix everything together and break down any lumps. Bring to a boil and let simmer gently for 15-20 minutes. Season to taste.
5) Meanwhile, cut the slices of bread into four to form large croutons. Bake for 10-15 minutes or until croutons become hard.
6) Serve the broth in individual bowls, add 2-3 croutons on top of each bowl and add about 1 teaspoon of Gruyère and 1 tablespoon of strong cheddar.
7) Bake on broil for 2-3 minutes. Keep on eye on the bowls, as they can easily burn.