Step 1

Melt the butter in a Dutch oven over medium heat. Add the onions and the salt, stir, and cover, letting the onions soften for 5 minutes.

Step 2

Remove the lid and let the onions caramelize until golden brown, stirring occasionally. (Adjust the heat if the onions are browning too quickly.) The caramelization process may take 45 to 60 minutes. Meanwhile, warm the stock in a saucepan over low heat.

Step 3

Once the onions are caramelized, add the wine and sherry to the pot and allow the liquid to come to a boil.

Step 4

Stir in the flour and smoked paprika and let thicken for a minute or two. Slowly add the warm broth, salt, and the pepper to the onions and boil uncovered for 10 minutes.

Step 5

Ladle the soup into the casseroles and cover the top with bread slices. Sprinkle each casserole generously with the cheese.

Step 6

Broil in the oven until the cheese melts and browns, watching carefully, a minute or two.