Heat oven to 375° F. On lightly floured board, roll out pastry dough 1/16-inch thick. Using 3-inch cutter, cut out 42 circles; re-roll scraps as needed.
Fit circles into bottoms and slightly up sides of lightly greased 2 ½ -inch muffin pans.
Meanwhile, fry bacon slices until crisp; drain and crumble or chop.
Chop mushrooms, sauté in butter until limp and liquid evaporates.
Combine bacon, mushrooms, green onion and cheese. Divide filling equally among muffin cups.
In large bowl, beat eggs, add sour cream and stir until smooth.
Spoon about 1 Tbsp. into each muffin cup. Bake until puffed and light brown, 20 to 25 minutes.
Cool in pans 5 minutes; lift out.
Serve warm or let cool on wire racks. If made ahead, wrap cooled quiches air tight, and refrigerate overnight. Reheat, uncovered, in 350° F. oven for about 10 minutes.