Slice the red onion very thin and sauté for about 5 minutes with a knob of butter, stirring frequently with a wooden spoon until transparent and soft.

Pour in the dark beer, let it simmer for a few minutes to make the alcohol evaporate then blend it with an immersion blender into a cream.

Add 3 tablespoons of cornstarch and stir with a whisk to remove any lumps and make it silky.

Stir in the hot chicken broth and the cheese, grated or cut into tiny cubes, whisking continuously.

Cook over low heat for about 10 minutes, stirring frequently with the whisk to prevent the cheese from sticking to the bottom of the pot. Season with salt and pepper to taste: the amount of salt needed will depend on the cheese you used.

When the soup is smooth and thick remove it from the heat, spoon the soup into individual bowls and serve it with a slice of grilled bread with a sprinkle of grated cheese on top.