How to cook
Sift the flour onto a sheet of paper. Put the butter in a pot with 2/3 cup (150 ml) of warm water. Bring to a boil slowly. Once it's boiling strongly, remove the pot from the heat, pour the flour in and stir vigorously. Transfer to a bowl and let cool for 10 minutes.
Meanwhile, heat the oven to 392°F (200 °C). Gradually add the eggs to the mixture. Add the chopped cheese, salt and cayenne pepper. Put 30 tablespoons of the mixture onto a moistened tray. Bake 30 minutes until they become crispy and golden.
Sprinkle 1 oz (25 g) of the grated gruyere over the hot eclairs. Let them cool.
Filling: Melt the butter in a pot. Sift the flour, fry for 1 minute and add the milk. Boil, add the grated cheese, sour cream, mustard, salt and pepper.
Heat. Cut the tops off the eclairs, fill them with the filling and close them.