Preheat oven to 400°. On a lightly floured surface, roll dough to a 1/8-in.-thick circle; transfer to a 9-in. pie plate. Trim pastry to 1/2 in. beyond rim of plate; flute edge. Refrigerate while preparing filling., In a large skillet, heat butter over medium heat. Add zucchini and onion; cook and stir until tender. Drain and cool slightly., Whisk eggs and seasonings until blended. Stir in cheese and zucchini mixture. Spread mustard over pastry shell; add filling., Bake on a lower oven rack until a knife inserted in center comes out clean and crust is golden brown, 35-40 minutes. If needed, cover edge loosely with foil during last 15 minutes to prevent overbrowning. Let stand 10 minutes before cutting.