You can make this a day ahead, if you like; the flavors will develop even more overnight. Reheat at 375°F, covered, for 20 minutes.
1 Tbs. plus 1/2 tsp. unsalted butter
1-1/2 cups fresh breadcrumbs
1/2 cup plus 2 Tbs. finely grated Parmigiano-Reggiano
1 Tbs. extra-virgin olive oil
1 large or 2 medium leeks (white and light-green parts only), halved and sliced 1/4 inch thick
2 tsp. minced fresh garlic
2 cups fresh corn kernels (from 3 to 4 large ears) I used 2 cups frozen
Freshly ground black pepper
2/3 cup heavy cream
3/4 cup lower-salt chicken or vegetable broth (I used water)
1/2 tsp. finely grated lemon zest
12 oz. peeled, seeded butternut squash, cut into 1/2-inch dice (about 2-1/2 cups)
8 oz. potatoes, cut into 1/3-inch dice (about 1-1/2 cups)
Position a rack in the center of the oven and heat the oven to 400°F. Rub a 2-quart shallow gratin dish with 1/2 tsp. of the butter.
In a 12-inch nonstick skillet over medium heat,
In a small bowl combine the breadcrumbs, 2 Tbs. of the Parmigiano, the olive oil, 1/2 tsp. of the oregano, and a large pinch of salt. Mix well.
Add 2 Tbs. butter to a large fry pan, medium heat. Add the leeks and a pinch of salt, and cook, stirring, until softened and just starting to turn golden, 6 to 7 minutes. Add the garlic and stir well. Add the corn, 1/4 tsp. salt, and a few grinds of pepper. Cook, stirring, until the corn has lost its raw look and is slightly shrunken, 2 to 3 minutes. Cool slightly.
Combine the cream and water or broth in a 2-cup liquid measure. Add the lemon zest, 1/2 tsp. salt, and a few grinds of pepper. Stir to mix well.
Add to the corn-leek mixture, the diced squash, potatoes, and remaining 1 tsp. of oregano. Toss lightly to combine. Spread the mixture evenly in the gratin dish. Sprinkle the remaining 1/2 cup Parmigiano on top. Stir the cream mixture one more time and pour it over everything as evenly as possible. (Be sure to scrape out any seasonings left in the cup.) Press down on the vegetables with a spatula so that the liquid surrounds them and everything is evenly distributed. Sprinkle the breadcrumb mixture evenly over all.
Cover with foil and bake for 20?minutes. Remove the foil and continue to bake until the crumb topping is deeply golden and the squash and potatoes are tender when pierced with a fork, about 25 minutes. The liquid should have bubbled below the surface of the vegetables, leaving browned bits around the edge of the pan. Let cool for 20 to 25 minutes before serving.