Spread the cubes of bread out on a baking sheet and allow them to dry overnight. Preheat your oven to 350 F. Heat the All-Clad 5 qt. Sautee over medium high heat and add the canola oil to the pan. When you see faint whips of smoke, add the onions. Allow the onions to caramelize then reduce the heat to medium. Continue cooking and stir occasionally. Periodically deglaze the bottom of the pan with a little bit of the water to release the caramelized glaze then allow the glaze to re-form. This process will develop both the color and sweetness associated with French onion soup. Continue the process until the onions have reduced significantly in volume, are very soft and have a deep mahogany color. Add the beef stock, vinegar and salt then remove from heat and allow to cool slightly.
While the onions are caramelizing, combine the dried bread and melted buttered toss so that the cubes are evenly coated. Spread the cubes out on a baking sheet and bake inside the oven until the cubes are golden brown, about 15 minutes. After the bread is toasted, return them to the mixing bowl and combine with the onion mixture, thyme leaves and beaten eggs. Gently fold the ingredients together with half of the cheese together distribute evenly and allow the bread to absorb the liquid.
Butter the All-Clad TK Oval Roasting Pan works great and pour in the stuffing mixture. Flatten and evenly distribute the ingredients then top with the remaining gruyere cheese and return to the oven to bake for 45 minutes or until the cheese is golden brown and bubbly.