Preheat oven to 375 deg F. Position rack in the center of the oven (you might have to remove the second rack to accommodate a pot with a lid). Meanwhile, heat oil in a 4 to 5 quart dutch oven (on the stove) on medium heat until shimmering. Add the onions along with a big pinch of salt, and stir well to coat with the oil. Cover and simmer for about 15 minutes, until the onions are soft and translucent.

When the oven finishes preheating, give the onions one final stir and spread them evenly on the bottom of the pan. Set the lid askew on the pot, and place in the oven. The onions will need to cook for about 1 1/2 to 2 hours. Check after 45 minutes, giving everything a good stir, resetting the lid again askew. Begin checking the onions about every 10 minutes at the 1:20 hour mark – you don’t want the onions to burn, but if they’re deeply golden brown with some burnt bits stuck to the bottom, they’re ready.

While the onions cook in the oven, soak the dried mushrooms in 2 cups of very hot (near boiling) water for 20 minutes. When the mushrooms are nice and soft, strain (and reserve) the liquid through a coffee filter. This is your mushroom broth.

Optional: for extra mushroom flavor and to give a nice body to the soup, blend the softened mushrooms with the mushroom broth, and set aside.

When the onions are ready, remove the pot from the oven, and immediately place on stove over medium heat. Add 1/4 cup water and begin deglazing the the pan, loosening any baked on bits from the bottom. Pour in the sherry and thoroughly deglaze the pan. Simmer until most of the sherry is evaporated (about 5 minutes). Add the vegetable and mushroom broths, plus the thyme and bay leaf. Reduce heat to medium-low, cover, and simmer for about a half hour. Remove the thyme sprigs and bay leaf.

To serve, preheat your oven broiler to low, positioning the rack so that there's plenty of room between the heating element and the rack. Ladle the soup into bowls, arrange the croutons on the surface (if using), and layer the Gruyere over the top. Place the bowls on a baking sheet, and broil until the cheese is melted, just a few minutes (if using bread bowls, take care not to scorch the bread - don't walk away from the oven!).