How to Make It
Preheat the oven to 425°. Coat four 12-cup mini-muffin pans with vegetable oil cooking spray. In a saucepan, simmer the potatoes in water over moderate heat until tender, 20 minutes. Let cool slightly, then peel and coarsely mash the potatoes.
Meanwhile, in a medium skillet, cook the sausage over high heat until browned, 8 minutes; coarsely crumble.
In a medium saucepan, bring the water, butter and salt to a boil. Remove from the heat and add the flour, stirring vigorously with a wooden spoon, until combined. Set the pan over moderately high heat and cook the dough, stirring, until it comes away from the sides of the pan, 2 to 3 minutes; remove from the heat.
Using a handheld electric mixer, beat the dough at low speed for 1 minute. Beat in the eggs one at a time at medium speed; beat in the Gruyère, Parmesan, thyme, rosemary and a pinch of pepper. Stir in the mashed potatoes and sausage.
Fill the muffin cups with the dough. Brush the tops with the egg wash and bake for 20 minutes, or until puffed and golden. Let the puffs cool slightly, then serve hot.