1/2 c. water
1/2 stick (4 Tblsp.) butter
1/8 tsp white pepper
1/2 c. flour
1/2 tsp. Dijon mustard
2 oz. gruyère cheese, finely grated
Preheat the oven to 375º and line a baking sheet with parchment paper (or grease it).
Heat the water, butter, and white pepper in a medium saucepan over medium-high heat. When the butter is melted and the mixture simmers, dump in the flour all at once and stir. Reduce the heat to medium and stir until the mixture pulls away from the sides of the pan1 to 2 minutes. Remove the pan from the heat and let the mixture cool slightly for a few seconds.
Add the eggs one at a time, beating well afer each. Beat in the mustard and all but 2-3 Tbsp. of the cheese. Drop the dough by teaspoons (or pipe through a pastry bag). Sprinkle with the remaining cheese.
Bake for about 25 minutes, or until they're puffed and golden brown. Serve warm. They will keep for 2 days in the fridge (well sealed), or frozen for up to 2 weeks. Reheat at 350º for 10-15 minutes.
Source: Great Bar Food at Home