This onion soup recipe has been a mainstay on the menu of my family's restaurant for over sixty years. The big difference in this Swiss recipe vs. a French onion soup recipe is that the broth in this recipe is thickened with roux and a dark beer is added to add another layer of flavor. It is a customer favorite and now I make it myself often on a winter day. Just a note about beef stock: I am now a convert about making my own stocks. It really does kick up all my recipes up a level. My brother is a chef and worked at a Michelin 3 star restaurant in Paris. He told me they would put their stock pots on the stove to simmer all night and start making sauces in the am. Really, overnight? So hey, that is what I do and I throw my vegetable scraps and bones with water in my stock pot on the lowest setting all night. In the morning, I have flavorful and clear stock and I did not work that hard. I freeze it in smaller containers. It is totally worth it. Try it with this recipe. —MyCommunalTable
Cook the onions with the butter in a small stockpot over medium-high until the onions are brown but not burned. This will take about 30 minutes. Stir regularly. Sprinkle in the paprika, then add the beef stock and bring to a boil.
Make the roux at the same time you are browning the onions by whisking the oil and flour together in a saucepan over medium heat. Watch carefully and stir consistently so as not to burn the roux. It will develop a popcorn smell first and then slowly turn brown. (You want a rich brown color for this recipe.) Stir the roux in the soup when done and add celery salt. Simmer for at least 2 hours.
Shortly before serving, sprinkle the slices of bread with Parmesan cheese and toast them in the oven. Add the beer and allow the soup to return to serving temperature. Remove from heat.
Ladle the soup into individual bowls or serving crocks, top with toasted brad and place a slice of Gruyere cheese over the top of each bowl. Place under the broiler for just a bit, watching carefully, until the cheese is melted, bubbly, and slightly brown.