Julia Child's French Onion Soup

Soup Preparation: In a cast iron stock pot or dutch oven over medium-low heat, add butter and olive oil and heat until butter is melted and blended with the olive oil. Add the onions and toss until they are evenly coated. Cover the pot and cook until the onions are very tender and translucent (about 20 minutes). Uncover, add the sugar and salt, stir well and increase the heat to medium/medium-high and cook, stirring often until the onions have browned and reduced significantly. When onions have reached a rich brown color and have reduced to less than a fourth of their original mass, reduce heat back to medium-low and sift flour over onions. Stir constantly until the flour browns, making sure not to scorch or burn the flour. (The flour will quickly turn from light tan to deep brown, so DO NOT step away from pot during this process!) Once the flour reaches a deep color, turn the heat back to medium-high and quickly add the wine to deglaze the pot. Add the cognac, beef bouillon stock, kitchen bouquet, thyme, sage, and bay leaf. Reduce heat to medium-low, cover, and simmer for 30 minutes. Remove cover and taste the soup, adding salt and pepper as needed. Topping Preparation: Heat oven to 350 degrees. Slice a Baguette into 1/4 to 1/2 inch slices. Lay the slices onto a sheet pan and drizzel the olive oil on both sides of the slices. Toast the slices 15 minutes on each side. Ladle the soup into a bowl, top with toasted baguette, top each bowl with 2 ounces swiss cheese and 1 ounce parmesan cheese. Place the bowl under the broiler until the cheese is melted and turns brown. Top the melted cheese with a pinch of salt and a grind of pepper.