Onion soup with Gruyere croutons
Instructions:
- Melt the butter in a large, heavy pot (not nonstick) over medium-high heat. When the butter stops foaming, add the onions, leeks, shallots, and garlic and a generous pinch of salt and stir to coat. Cover and cook, stirring often, until the vegetables soften and reduce to about half of their original volume, about 15 minutes. Uncover, reduce the heat to medium-low, and cook until the vegetables are very soft and deep golden brown, about 30 minutes, stirring occasionally to scrape up the browned glaze on the bottom of the skillet. Reduce the heat and stir more frequently if the vegetables start to scorch or get crunchy.
- Add the wine and simmer until it cooks away, about 3 minutes. Add the beef stock, chicken stock, and mustard. Bring to a boil, reduce the heat, and simmer for 15 minutes. Add the Cognac and simmer 5 minutes. Season with the salt and pepper. Cool to room temperature, cover, and refrigerate overnight. When ready to serve, reheat over medium heat, check the seasoning, and stir in the vinegar.
- To make the Gruyere croutons, preheat the broiler. Arrange the baguette slices in a single layer on a rimmed baking sheet and brush them lightly with oil. Broil until the bread is lightly toasted. Remove the pan from the oven and top each piece with a little mound of Gruyere. Place back under the broiler until the cheese melts.
- Serve the hot soup with the Gruyere croutons on the side.
- Variation: You can also use the method described in Slow Cooker Caramelized Onions (recipe follows) to cook the onions, leeks, shallots, and garlic. When done, transfer into the soup pot and pick up the recipe at Step 2.
- Make-ahead note: This soup tastes best made 1 day ahead. Cool, cover, and refrigerate. Check the seasoning and reheat gently before serving.