1. Fry bacon over medium heat in a medium frying pan. Transfer to a bowl with a slotted spoon and pour out all but 1 tbsp. bacon fat from frying pan. Add butter, swirl to melt, and add leeks, garlic, salt, and a few grinds of pepper. Cook, stirring occasionally, until leeks are tender-crisp, about 4 minutes. Pour in white wine and stir in bacon. Lower heat to low and cook, covered, about 5 minutes, or until leeks are tender and wine is evaporated. Let cool to room temperature.
2. Preheat oven to 400°. On a floured work surface and with a floured rolling pin, roll pastry sheet out to a thickness of 1/8 in. Cut pastry into rounds with a buttered and floured 2-in. biscuit or cookie cutter with fluted edges and arrange rounds 1/2 in. apart on 2 baking sheets.
3. Press insides of each round with your finger to make a depression, leaving a 1/4-in. rim. Poke center of each round once with a fork and spoon a scant tsp. of mushroom filling onto centers. Whisk egg, cream, and nutmeg together well and set aside.
4. Bake pastries until golden brown and beginning to puff, 8 to 12 minutes. Remove from oven and lower heat to 350°.
5. With end of a small funnel, carefully punch down centers of pastries while spooning about 1/2 tsp. egg-cream mixture into top of funnel (to help guide the liquid) and into pastry (spoon in more if the pastry will accept it without overflowing). Bake pastries another 10 to 15 minutes, or until deep golden brown.
Note: Nutritional analysis is per puff.