Preheat the oven to 375 F.
Forming the Dough - Spray cooking spray in a pie pan. Add flour to the rolling pin and loosely roll the pie dough (that has been rolled into a circle) over the rolling pin. Lay the rolling pin over the pie pan and unroll the pie crust over the pan leaving the extra dough to hang evenly off the sides of the pan. Set aside.
For the Eggs - In a medium-size bowl, add the eggs and milk. Whisk together and transfer the egg mixture to a large pouring measuring cup (for easy and even pouring over the filling ingredients).
Filling the Pie Shell - In the pie shell, add the potatoes in an even layer, half of the egg and half of the cheese.
Then add an even layer of chopped ham, spinach, green onion, the rest of the cheese. Evenly over the top, pour in the remaining egg.
For the Crust Edges - Fold the excess dough that’s hanging over the edges of the pan inwards towards the middle. Lay slices of tomato in the middle of the pie (looks so pretty after it bakes).
For the Egg Wash - in a small bowl, add the egg and water. Whisk together and brush over the folded edges of the pie crust.
Bake in a preheated oven until the pie crust turns a golden color and the egg filling is thoroughly cooked (about 50 to 55 minutes).
Serve while warm or store in the refrigerator and warm each piece in the microwave just before serving.