Remove top sheet and invert dough into a 10-inch tart pan. Lift edges of the dough, push into the sides of the pan, using excess dough to build up the edge slightly. Peel off wax paper and trim dough to the edge of the pan. Prick bottom and sides and chill for one hour. Remove and cool.
Slice tomatoes into thin rounds and place in marinade.
Let stand one hour. Paint crust with mustard. Scatter mushroom and onions over the top. Sprinkle with parmesan cheese. Drain tomato slices and arrange in a layered circular pattern over parmesan.