In a food processor pulse flour and butter until the butter is finely cut in. Add the salt and Parmesan and pulse a few times to mix. Add the cold water slowly and pulse just until dough comes together into a ball. Shape the dough into a flat disc, wrap in plastic wrap and refrigerate 30 minutes.
Rinse the food processor and pulse the onion to a rough chop. Heat oil in a medium skillet and saute onion for 2-3 minutes. Pulse summer squash in food processor til rough chopped. Do not over process. Add summer squash and cook about 5 minutes more. Add corn and cook for 3 minutes more . Add salt and pepper and set aside to cool.
Remove pastry from fridge. Spray 9" spring form pan with cooking spray. Roll dough on floured surface until it is large enough to fill a 9" spring form pan base and sides. Press dough into pan and up the sides of the pan. Trim off any excess dough above the sides. Put back in the fridge for 20 minutes.
Preheat oven to 375 degrees.
Prick bottom of dough with a fork. Cover dough with a piece of tinfoil and fill with pie weights or dry beans or rice for blind baking. Bake tart shell for 20 minutes, remove pie weights and bake 15 minutes more.
Reduce heat to 325 degrees. Set tart shell aside.
Whisk eggs, cream, salt, pepper together and stir into cooled corn mixture. Spread ½ the cheese on the bottom of the tart shell, pour in the corn filling and top with the remaining cheese.
Bake 50-60 minutes until top is golden and center is set. Let rest at least 10 minutes before serving.
Tart is delicious hot or at room temperature.