Making the Choux Pastry
Preheat your oven to 375°F / 190°C and lightly butter several large baking sheets.
In a medium saucepan, combine the water, butter, salt, and sugar.
Bring the mixture to a rolling boil over medium-high heat, ensuring the butter is fully melted.
Reduce the heat to low and add all the flour at once.
Stir vigorously with a wooden spoon until the mixture pulls away from the sides of the pan and forms a smooth ball of dough.
Continue to cook, stirring constantly, for another 3-4 minutes to dry out the dough slightly.
Remove the pan from the heat and let the dough cool for 5 minutes. It should be warm, not hot.
Transfer the dough to the bowl of a food processor fitted with the steel blade. Pulse for about 20 seconds to break it up.
Add the eggs one at a time, processing well after each addition until the dough is smooth, thick, and glossy.
Transfer the finished dough to a pastry bag fitted with a round tip, or use two teaspoons. Pipe or spoon small, walnut-sized balls onto the prepared baking sheets, leaving about 1½ inches of space between them.
Bake for 20 to 30 minutes, until the puffs are beautifully puffed up and a light golden brown.
Do not open the oven door for the first 15 minutes!
Once baked, use the tip of a sharp knife to poke a small slit in the side of each puff.
Turn off the oven and return the puffs to the warm oven for 15-20 minutes to dry out the insides. This is the secret to a crisp shell!
Crafting the Filling & Assembly
In a large bowl, gently combine the chopped hard-boiled eggs, parsley, marinated mushrooms, and cooked asparagus.
Drizzle the melted butter over the mixture and toss lightly to coat.
Season with salt, white wine vinegar, and a generous amount of freshly ground black pepper.
Stir carefully until everything is well combined.
Taste and adjust seasoning if needed.
To assemble, carefully slice off the top third of each cooled puff, leaving it slightly attached like a hinge. Spoon about a teaspoon of the savory filling into each puff. Press the top down gently and serve immediately with cocktails or chilled white wine.