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.
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, cook the bacon until crisp, about 8 minutes. Transfer the bacon to paper towels. Reserve 2 Tbs. of the fat in the skillet; discard the remainder. When the bacon is cool, crumble or mince it.
In a small bowl, combine 1 Tbs. of the crumbled bacon with the breadcrumbs, 2 Tbs. of the Parmigiano, the olive oil, 1/2 tsp. of the thyme, and a large pinch of salt. Mix well.
Add the remaining 1 Tbs. butter to the skillet with the bacon fat and melt over 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 chicken 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.
In a large bowl, combine the remaining bacon, the corn-leek mixture, the diced squash, potatoes, and remaining 1 tsp. thyme. 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.
nutrition information (per serving):
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Photo: Scott Phillips