Place parsley in work bowl and process until finely chopped. Remove to side dish. With machine in on position, drop shallots through feed tube. Stop machine and add onions to work bowl. Process until onions are finely chopped using pulse action.
Melt butter in large skillet and cook onions and shallots slowly.
Process mushrooms 2 cups at a time using quick pulse actions, scraping work bowl occasionally, until mushrooms are chopped quite finely, but not pureed. Add to onion in skillet as soon as onions are soft. Cook mushrooms and onions until mixture is quite dry, stirring frequently. Add flour and mix well so flour will cook slightly. Add cream, salt, pepper, and nutmeg and cook until quite thick. Remove from heat and allow to cool slightly. Stir in egg yolks and chopped parsley.
Spoon mixture into prepared pastry shell. Roll out pastry trimmings and cut with crinkled pastry wheel into 8 or 10 long strips. Make a lattice work on top and brush with egg glaze.
Heat oven to 450 degrees and place tart on baking sheet in bottom of oven. Bake over hot bottom element 10 minutes. This will ensure adequately cooked bottom crust. Move tart to middle shelf, reduce heat to 400 degrees and cook 25 to 35 more minutes or until puffed and browned. Filling should spring back slightly when touched lightly with finger.
Serve warm or at room temperature.