Preheat oven to 400 degrees F (200 degrees C).

Grease and lightly flour a large baking sheet.

Combine water, 1/2 cup butter, and salt in a saucepan.

Bring to a boil over medium heat.

Add flour all at once and remove from heat.

Stir vigorously with a wooden spoon until smooth and a dough ball forms that leaves the side of the pan.

Add eggs one at a time, stirring until smooth after each addition.

Spoon dough into a large pastry bag fitted with a fluted nozzle.

Pipe dollops of dough (about 1 tablespoon each) onto the prepared baking sheet, leaving room for spreading.

Bake for 20-25 minutes, until golden brown.

Let cool completely.

Melt 2 tablespoons of butter in a large skillet.

Add minced onions and mushrooms; sauté over medium-low heat until tender.

Stir sour cream into the onion mixture and let cool.

Cut baked pastry swirls in half horizontally.

Using a spoon or small paring knife, remove any soft dough from inside the puffs, leaving a hollow shell about 1/4 to 3/8 inch thick.

Spoon about 1 tablespoon of the sour cream mixture into the bottom portion of each swirl.

Replace the top portion of the swirls.

Preheat oven to 350 degrees F (175 degrees C).

Bake on a large baking sheet for 10 minutes, or until heated through.