Dice bacon.
Peel and finely chop onion.
Fry bacon and onion in a medium skillet until bacon is crisp.
Add mushrooms, tomato paste, celery salt, pepper and butter; fry until liquid evaporates.
Remove from heat.
Preheat oven to 425°F (220°C).
On a floured surface, roll out pastry dough 1/8 inch (3 mm) thick.
Cut into eight to ten 4-inch (10-cm) circles.
Including as little fat as possible, place a spoonful of filling on 1 half of each circle.
Fold over other half to cover, making a half-moon shape.
Using the prongs of a fork, press edges together to seal.
Brush pasties with beaten egg.
Sprinkle a baking sheet with cold water.
Arrange pasties on dampened baking sheet.
Bake 15 to 20 minutes or until puffed and golden