Sweet onions, such as Vidalia or Walla Walla, will make this recipe overly sweet. Be patient when caramelizing the onions in step 2; the entire process takes 45 to 60 minutes. Use broiler-safe crocks and keep the rim of the bowls 4 to 5 inches from the heating element to obtain a proper gratinée of melted, bubbly cheese. If using ordinary soup bowls, sprinkle the toasted bread slices with Gruyère and return them to the broiler until the cheese melts, then float them on top of the soup. We prefer Swanson Certified Organic Free Range Chicken Broth and Pacific Beef Broth. For the best flavor, make the soup a day or 2 in advance. Alternatively, the onions can be prepared through step 1, cooled in the pot, and refrigerated for up to 3 days before proceeding with the recipe.