Think of this as the Italian version of French onion soup. When we were testing this recipe in my kitchen, I had a box of mushrooms in my refrigerator and wanted to use them up. When we sat down to lunch that afternoon, and Tanya tasted the soup, she said, "I don't know why anyone hasn't thought of this before! Onion soup should always have mushrooms in it!"
Preheat the oven to 425 F.2.
Heat the butter and olive oil in a low, wide Dutch oven over medium-low heat. Add the onions, cover and cook, stirring occasionally, until golden, 15 to 17 minutes. Add the mushrooms. Increase the heat to medium. Cook and stir until the mushrooms and onions are dark golden, about 10 minutes more. Add the thyme and season with salt and pepper. Add the white wine, and summer until it's almost completely reduced, about 2 minutes. Add the stock and simmer until the onions are very tender and the soup is thick, 12 to 15 minutes.3.
Toast the bread on the oven rack until just golden, 3 to 4 minutes. Toss the fontina and Grana Padano together in a medium bowl. Set the pieces of bread on top of the soup, starting at the edges, then filling in the middle. Sprinkle with the cheese mixture. Bake in the oven until the cheese is browned and bubbly, about 20 minutes. Spoon the bread and soup into wide soup bowls to serve. This soup can also be poured into individual ramekins, then topped with the bread and cheese and baked.
From "Lidia's a Pot, a Pan, and a Bowl" by Lidia Matticchio Bastianich and Tanya Bastianich Manuali. Copyright © 2021 by Tutti a Tavola, LLC. Excerpted by permission of Alfred A. Knopf, a division of Penguin Random House LLC. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.