Preheat oven to 400°F.
Insert the cloves into the outer skin of the onion and place into a small saucepan with the cream, rosemary, garlic, peppercorns and ½ teaspoon kosher salt. Warm over medium-low heat for 15 minutes, stirring often to ensure the cream does not boil. Remove from the heat and strain the cream into a bowl. Set aside.
Pour enough of the cream into a baking dish to just cover the bottom (13- by 9-inch or 10- by 10-inch dishes work well). Place the thinly sliced potatoes in a single, even layer in the bottom of the dish. Pour an additional ⅓ cup cream over the potatoes and then add ¼ of the parmesan cheese along with a pinch of salt and a pinch of black pepper. Next, add a layer of summer squash, followed by an additional ⅓ cup cream, ¼ cup parmesan and a pinch of salt and pepper. Repeat with the zucchini and then the potatoes again, continuing the process until all of the potatoes, squash and zucchini have been used, creating approximately 6 layers. Pour the remaining cream and parmesan over the top layer and then spread the tomatoes out over top.
Combine the panko, red pepper flakes, garlic powder and dried oregano in a small bowl along with ¼ teaspoon salt and ⅛ teaspoon black pepper. Sprinkle the breadcrumbs over the tomatoes and drizzle lightly with olive oil.
Bake the gratin in the oven until brown and bubbly and a knife inserts easily from the top of the gratin through the potatoes, squash and zucchini, approximately 1 hour. Allow to sit for 3 minutes before serving.