In 10-inch skillet, heat oil and butter over medium-high heat. Add onion; cook 5 minutes, stirring occasionally, until onion begins to brown. Reduce heat to medium-low; cook 20 to 25 minutes, stirring frequently, until browned and tender.
Meanwhile, heat oven to 450°F. Unroll pie crust on work surface. Roll pie crust with rolling pin to 12 inches in diameter. Gently press crust in bottom and up side of ungreased 10-inch tart pan with removable bottom. Trim off excess crust. Prick bottom and side of crust with fork. Bake 10 minutes. Remove from oven. Reduce oven temperature to 400°F.
Sprinkle bottom of partially baked crust with 2 tablespoons of the Parmesan cheese. Using slotted spoon, spoon onions evenly over bottom of crust. Top with tomatoes and zucchini, arranging a circle of tomato slices around outside edge, a circle of zucchini slices next to the tomatoes and a circle of tomato slices in the center.
In small bowl, beat whipping cream and egg with wire whisk. Pour over vegetables. Top with Gruyère cheese and remaining 2 tablespoons Parmesan cheese.
Bake at 400°F 15 minutes. Top with green onions; bake 10 to 15 minutes longer or until filling is set and crust is golden brown. Cool 5 minutes before removing side of pan.