Roll dough between two pieces of parchment paper to make a circle about 13 inches in diameter. Line a 10-inch tart pan and trim top edges with a knife. Fit parchment paper into shell and fill with weights (lightly oiled dry beans work well).
Preheat oven to 375F. Cook tart crust for 10 minutes. Cool slightly. Remove paper with weights. Cook 10 minutes longer until lightly colored. Cool.
Slice mushrooms, chop shallot, and cut pancetta (if using) in 1/8-inch dice. Heat olive oil over medium-high heat in a large skillet. Cook shallot and pancetta until golden. Scrape into a bowl.
In a medium bowl, whisk eggs with cream. Season with ½ teaspoon salt and ¼ teaspoon pepper.
Spread mushroom mixture over pastry shell. Sprinkle with sage leaves. Pour egg-cream mixture over mushrooms and tilt to distribute evenly. Top with Fontina and pecorino. Bake until filling is just cooked and cheesy top is browned, about 25 minutes. Serve warm or at room temperature.