I cut this recipe out of a newspaper over 40 years ago. I do not know which newspaper. Washington Post, probably.
2 tablespoons butter
1 small onion, finely chopped
1 box (12 ounces) mushrooms, very finely chopped
2 Tablespoons flour
1 1/2 cup light cream
pinch of grated nutmeg
pinch of ground allspice
salt and freshly ground black pepper, to taste
3 egg yolks
1 egg, beaten to mix, for glazing pastry
For filling, melt butter in a skillet and cook onion for 2 minutes. Add mushrooms and cook a few minutes until moisture is released, then turn up heat and cook until all moisture is evaporated, stirring constantly at the end. Add flour and cook 1 minute. Add light cream and bring to a boil, stirring. Add nutmeg, allspice and salt and pepper to taste, and simmer 3 minutes. Remove from heat, let bubbles subside and add the egg yolks, 1 at a time. Stir in parsley* and set aside to cool.
Set oven at 400 degrees F. Roll out half the pastry on a lightly floured board and line a 10 inch tart pan with removable base. Trim off excess: prick bottom a couple dozen times and spread cooled filling in the shell.
Roll our remaining pastry and use a ravioli cutter or knife to cut in to 1/2 inch strips. Make a lattice on the filling, setting strips 1/2 inch apart and making the second set of strips diagonal, not perpendicular, to the first. Glaze lattice with beaten egg.
Set tart on a baking sheet and bake it on the bottom rack of the oven 20 minutes. Transfer to top oven rack and continue baking another 10 minutes, or until pastry is golden brown.
*The parsley is not in the list of ingredients. Surprise!
I wish I would make this every year. Or at least once a year. I have tried it with various mushrooms, white, crimini, a mix, some portobellos. All of the times given must be for a much better stove than mine, because my times are longer for the moisture release. The baking time works in my oven, though I think I have ignored switching the levels. I don't always glaze it. I do not have a 10 inch tart pan with a removable base. I like the lattice directions, be sure to do diagonal, not perpendicular. Yum.