For the soup

1. Prep onions. Cut the onions in half lengthwise (stem to root), remove the onion skins and discard. Slice the onion halves into 1/8 to 1/4 inch thick slices, discarding the root ends. Place all the onion slices into the insert of a 6-quart slow cooker.
2. Cook onions. Add the melted butter and salt; toss to coat evenly. Cover the slow cooker and cook on high heat for 8-9 hours, stirring occasionally. At this point, the onions should be a deep brown color and there should be only a little bit of liquid in the pan.
3. Make the soup. Turn the slow cooker to high heat. Add the sherry and stir until most of the liquid is absorbed. Add the chicken and beef broths and stir to combine. Let soup simmer for 15-30 minutes and is heated through. Taste soup and season with salt and pepper to taste.

For the cheese crostini

1. Toast bread. While the soup is simmering, heat oven to 400 degrees F. Brush olive oil on baguette slices and place slices on a baking sheet in a single layer. Bake until bread is crisp, and just golden at the edges, 6-10 minutes.
2. Melt cheese. Sprinkle cheese on the baguette slices and return pan to the oven. Continue baking until cheese is melted.
3. Serve. Ladle hot soup into serving bowls and serve with cheesy baguette slices.