From Sarah Steedman
Sarah picked this soup because she wanted to make something seasonal, and while not a lot is in season in March, you may well have some onions lurking in your root cellar — if you’re into that kind of thing. It’s adapted from a recipe in the March 2010 issue of Bon Appetit.
8 tablespoons butter, divided
24 cups thinly sliced onions (about 5 3/4 pounds)
8 cups (or more) low-salt chicken broth
1 1/2 cups multigrain bread, cut into ½” cubes
3/4 cups coarsely grated gruyere cheese
3 tablespoons fresh thyme leaves, divided
1 1/2 tablespoons (or more) white wine vinegar
Melt six tablespoons butter in an extra-large heavy pot over medium-high heat. Add onions, sprinkle with salt and pepper and sauté until onions begin to soften, 15 to 18 minutes. Reduce heat to medium and sauté until onions are very tender, stirring often and adjusting heat as needed, one hour longer.
Preheat oven to 375 degrees. Toss bread cubes with cheese, one tablespoon thyme and two tablespoons melted butter, season with salt and pepper and spread on a rimmed baking sheet. Bake until cheese has melted and bread is lightly crisp, about 10 minutes. Let cool.
Add eight cups broth to onions in pot and bring to boil. Add two tablespoons thyme. Reduce heat and simmer 25 minutes to blend flavors. Cool slightly. Working in batches, puree soup in blender until smooth. Return to same pot and add more broth by 1/4 cupfuls to thin soup to desired consistency. Season soup to taste with vinegar, salt and pepper.
Divide warm soup among eight bowls. Sprinkle each serving with croutons.