Heat your oven or toaster oven to 400°F. Ensure the rack is in the middle or lower.

Grate the cheese and set it aside.

Beat the yolk with the mustard, cayenne and salt with a fork. Stir in the grated cheese.

Whisk the egg white by hand in a mixing bowl until firm peaks are formed. Fold it into the cheese, then heap the mixture onto the bread you’re using.

Slide into the toaster oven and bake until puffed and golden, about 10 minutes. When done, remove from the oven and dust lightly with smoked paprika or plain paprika. Serve whole or cut into pieces.