Pastry puffs are delectable on their own, but Italian bacon makes them downright addictive.
Heat oven to 375 degrees. Combine butter, salt, and 1 cup of the water in a medium saucepan over medium-high heat; bring to a boil, and immediately remove from heat. Quickly stir in the flour with a wooden spoon until combined.
Return to medium heat; cook, stirring vigorously, until mixture pulls away from sides and a film forms on bottom of pan, about 3 minutes. Remove from heat; transfer to a bowl to cool slightly, about 3 minutes. Add 4 eggs, one at a time, mixing well before adding the next one. Stir in cheese and black pepper.
Whisk remaining 1 egg with the 1 tablespoon water in a small bowl. Line two large rimmed baking sheets with parchment paper. Transfer dough to a pastry bag fitted with a 3/8-inch plain tip. Pipe dough into 1-inch rounds about 1 inch apart. Smooth with a moistened finger to ensure even rising. Brush with egg wash. Bake until puffed and golden brown, about 25 minutes. Gougeres should feel light and airy. Transfer baking sheets to wire racks to cool completely. Store up to 1 week in airtight container.
To assemble, slice gougeres in half crosswise. Spoon on 1/2 teaspoon tomato relish; top with pancetta and micro arugula. Replace the top, and serve.