The ultimate cold weather dish—soupe à l’oignon gratinée—was the pièce de résistance of a popular winter cooking class called “Midnight in Paris” that I taught several years ago. This recipe is based loosely on the first onion soup I ever made from Julia Child’s Mastering the Art of French Cooking, Volume I. The main difference is that I suggest using a quick short-cut beef stock instead of Julia’s homemade stock.
This is a soup where the quality of ingredients used for the topping matters. An aged Gruyère and slices of a good crusty baguette will add immeasurably to the dish’s success. One last tip direct from Julia: She suggests stirring some small strips of Gruyère into the soup before adding the toasted bread slices. Those little strips melt as the soup simmers in the oven, melding beautifully into the onion broth.
Recipe excerpted from Soup Nights: Satisfying Soups and Sides for Delicious Meals All Year by Betty Rosbottom, Rizzoli New York, 2016. Click here to purchase your own copy.
In a 5-quart heavy pot (with a lid) over medium-low heat, heat the butter and oil. When hot, add the onions. Cover and cook, stirring frequently, 15 minutes.
Remove the lid and raise the heat to medium. Stir in 1 teaspoon salt, the sugar, and the flour. Cook, stirring constantly, scraping the bottom of the pan so that the flour does not burn, until the onions are rich golden (like the color of light brown sugar), 35 to 40 minutes or more. (While you are cooking the onions, the flour will start to darken too and the onions will cook down considerably. That’s okay.)
When the onions are done, add the simmering stock and 1/2 cup of the wine. Season the soup with salt and pepper, and a pinch or two of extra sugar if desired. Simmer, partially covered with the lid set ajar, 40 minutes more. With a large spoon, skim off any foam that forms. Add the remaining 1/4 cup wine and season the soup again with salt and pepper. (Soup can be prepared three days ahead. Cook to this stage, then cool, cover, and refrigerate. Reheat over medium heat.)
While the soup is simmering, prepare the baguette slices and the cheese topping. Arrange a rack at center position of the oven and preheat to 350°F.
Brush the baguette slices generously on both sides with olive oil and arrange on a rimmed baking sheet. Bake until slices are crisp, 4 to 5 minutes per side. Remove and cool. (Baguette slices can be prepared two days ahead; store in an airtight container at room temperature.) Retain oven temperature.
Arrange 6 ovenproof soup bowls or ramekins on a rimmed baking sheet and fill them 3/4 full with the hot soup. Divide the slivered cheese among the bowls. Float 2 to 3 baguette slices on top of each serving, and sprinkle generously with some grated cheese. Depending on the size of your bowls or ramekins, you may have some soup, cheese, or croutons left over.
Bake the soups until the cheese has melted and is lightly browned, 15 minutes. Watch constantly. If desired, run under a hot broiler to brown more, 1 to 2 minutes.