1 cup water
6 tablespoons unsalted butter — cubed
1/2 teaspoon kosher salt
1 1/4 cups all-purpose flour — (about 5 3/8 oz.)
4 large eggs
6 ounces Cheddar cheese — finely shredded (about 1 cup)
2 tablespoons diced pimentos — drained
1 teaspoon Dijon mustard
Preheat oven to 400°F. Heat water, butter, and salt in a saucepan over medium until butter is melted, about 3 minutes. Remove from heat. Add flour all at once, and stir with a wooden spoon until combined.
Return to heat, and cook mixture, stirring constantly, until dough forms a ball and holds together, about 3 minutes. Continue to cook, stirring constantly, until dough is no longer tacky, about 2 minutes more. Remove from heat, and let dough cool slightly, about 5 minutes.
Add eggs, 1 at a time, stirring quickly and incorporating completely after each addition. Stir in cheese, pimentos, and Dijon until combined.
Transfer dough to a pastry bag with the tip cut off, and pipe 2-inch rounds onto a baking sheet lined with parchment paper. Spray the back of a spoon with cooking spray, and smooth the rounds so they have no peak. Bake in preheated oven until puffed and golden, about 25 minutes.