Roll out the pie dough and fit it into an rectangle baking tray that's been lined with parchment paper; set aside.

Using a mandoline slicer, slice the zucchini into rounds.

Slice the grape tomatoes in half; lengthwise.

Set the vegetables aside while you prepare the onions.

Heat the butter in a heavy skillet.

Add the sliced onions and cook until they are soft and golden brown; about 20 minutes.

Spread the mozzarella cheese on top of the pie dough.

Add the cooked onions on top of the cheese.

Arrange the sliced zucchini and then the tomatoes on top.

Sprinkle the Parmesan cheese on top of the vegetables and then the basil and parsley.

Brush the beaten egg and milk mixture over the entire pie crust.

Bake in a preheated 425 degree oven for about 20 minutes or until the crust is golden brown.