Preheat oven to 425 degrees. Rinse, pat dry and slice mushrooms (makes about 4 cups); set aside. Roll pastry 1/8-inch thick; fit into 9 inch pie pan; turn under and flute edges; prick with fork tines. Bake for 5 minutes; remove from oven.

In a large skillet, melt 1 tbs. of the butter. Add onion; saute until tender, about 5 minutes. Remove from skillet, set aside. Melt remaining 2 tbs. of butter in skillet; add mushrooms. Saute until tender, about 5 minutes; set aside.

In a medium bowl, lightly beat eggs. Add sour cream, salt, tarragon, and black pepper; mix well. Reserve 1/3 of the mushrooms; add remaining sauteed mushrooms and onion to sour cream mixture; mix well.

Pour into partially baked pie shell. Sprinkle with cheese. Bake until a knife inserted in the center comes out clean, 20 to 25 minutes. Heat reserved sauteed mushrooms until hot. Arange on top of pie. Let pie stand 5 minutes before cutting.