Gooey, Cheesy Baked Onion Soup

Why I Love This Recipe
Recipe courtesy of 'Once Upon A Tart' by Frank Mentesana and Jermoe Audureau.
This is best onion soup recipe I've ever tried, but of course, my family loves all things cheesy, so it's not hard to see why!
Ingredients You'll Need
2 big yellow onions, quartered and thinly sliced
2 red onions, quartered and thinly sliced
4 leeks, root end and dark-green tops removed, sliced 1/4 inch thin, and washed
1 tablespoon unsalted butter
1 tablespoon olive oil
2 tablespoons unbleached all-purpose flour
8 cups vegetable or chicken stock
1 tablespoon Worcestershire sauce
1 teaspoon salt
1/2 teaspoon freshly ground black pepper
1 day-old baguette
Grated Gruyere cheese
1/2 cup grated Parmesan cheese
Directions
Saute the yellow and red onions and the leeks in the butter and olive oil in a large soup pot over high heat for 5 to 10 minutes, stirring occasionally to keep the onions from sticking to the bottom of the pot and burning.
When the onions have begun to soften slightly and reduce in volume, 5 to 10 minutes, lower the heat to medium and continue to sauté them for about an hour, until they are caramel in color and almost melted. You don't want the onions to burn, so, if it looks like they're heading in that direction, lower the flame even more, or splash a little bit of water over them -” this buys you some cooking time.
Sprinkle the onions with the flour, stirring all the while so the flour doesn't clump. Sauté for another 5 minutes to cook off the floury taste.
Pour the stock over the onions and bring it to a boil over high heat. Reduce the heat and simmer for 30 minutes. Stir in the Worcestershire sauce, salt, and pepper.
Preheat your oven to 350 degrees. Slice the stale bread, brush the slices with olive oil, scatter them on a sheet pan, and bake until the bread dries out and the edges are golden brown.
Just before you're ready to serve the soup, whisk in the Parmesan cheese. It's important to whisk in the cheese at the last minute, or else the cheese will fall to the bottom of the pot and burn. Ladle the warm soup into heatproof bowls, and lay a slice of the baked bread over each bowl. Sprinkle a layer of Gruyère cheese over the bread, and place the crocks under the broiler until the cheese bubbles and browns.