Preparation

• In a 4-quart saucepan, bring the broth, chopped onion, and porcini to a boil. Reduce to a simmer and cook for 15 minutes. Strain through a fine-mesh sieve lined with damp paper towel into a large bowl. Clean the saucepan and return the broth to the pan.
• Meanwhile, heat the butter and oil in a 12-inch skillet over medium-high heat until the butter browns, about 2 minutes. Add the sliced onions, thyme, and paprika. Cover and cook until the onion is soft and golden, stirring occasionally, about 10 minutes. Add the sherry and stir to scrape up any browned bits on the bottom of the skillet. Add the contents of the skillet to the broth. Add 1-1/2 tsp. salt and bring to a boil. Cook for 2 minutes and then turn off the heat.
• Position a rack 6 inches from the broiler and heat the broiler on high. Place four broiler-safe 12-oz. soup crocks or deep bowls on a rimmed baking sheet. Divide the soup among the bowls and top each with a slice of baguette. Evenly distribute the cheese over each. Broil until the cheese is melted and bubbling, 1 to 2 minutes.