Preheat oven to 375 degrees.

Using a mandolin, slice the zucchini, the yellow squash, the eggplant and the tomatoes 1/4 inch thick. Reserve.

In a skillet, heat the olive oil and add the onions and some salt. Cover and cook over medium heat until they are soft (about 10 minutes). Remove the lid and continue cooking until the onions are golden and caramelized. That should take 10 more minutes.

Dust your counter with flour and roll your puff pastry to about a 11" square (or 11"X15" rectangle if you don't have a tart pan).

Transfer to a tart pan, making sure you press the dough against the ridges on the side. (If you don't have a tart pan, just transfer the dough to a baking sheet covered with parchment paper)

Prick the dough, all over, with a fork.

Spread the crushed tomatoes evenly over the pastry and sprinkle some salt.

Spread the caramelized onions on top of the crushed tomatoes.

Layer the zucchini, eggplant, squash and tomatoes in an overlapping spiral. Sprinkle salt and pepper.

Drizzle with some olive oil.

Top with 1/2 cup of freshly grated Parmigiano Reggiano.

Bake for 30 minutes or until the pastry is golden brown and the cheese has melted.