Squash casserole is a Southern summer must. My go to has been a classic version, rich with cheese and sour cream, and I adapted that for a pimento cheese version in my first book, Pimento Cheese the Cookbook. Most southerners deal with an overabundance of summer squash, whether it’s from friends who garden or the irresistible urge to buy the locally grown, beautiful sunshine yellow vegetable at the farmers market. So we are always looking for new and interesting ways to serve fresh squash. This gratin has the beautifully creaminess we love in squash casserole, but is lighter and very fresh and bright. My mom told me this was the best thing I’ve made recently, and what better endorsement could there be.
I like that the leeks in this dish complement the squash without overwhelming – the brilliant sunny taste of the squash shines through. Thyme is my favorite herb with squash, but feel free to branch out with marjoram or oregano. Nutty gruyere cheese and a light hit of breadcrumbs make a rustic topping, but you could also use fontina or swiss.
- 2 large leeks, white and lightest green parts only
- 2 pounds yellow summer squash (about 4 medium)
- ½ cup (1 stick) unsalted butter, divided
- 2 teaspoons fresh thyme leaves (stripped from the stalks)
- 2 Tablespoons all-purpose flour
- 1 teaspoon kosher salt
- 1 teaspoon black pepper
- 1 cup milk, heated in the microwave or a small pan until hot to the touch
- ¾ cup grated gruyere cheese
- ½ cup bread crumbs
- Slice the leeks in in half, then slice the leeks into thin half-moons. Place in a colander and rinse well, then leave to drain for a few minutes. Thinly slice the squash – I use a mandoline, but the slicing blade of a food processor also works, or a knife and a little patience.
- Melt 6 Tablespoons of the butter over medium heat in a large, deep-sided sauté pan or a Dutch oven (you need room to stir the squash around). Add the leeks with some water clinging to them and stir to coat with the butter. Cook, stirring frequently, until the leeks are soft and glassy, do not let them brown, about 15 minutes. Add the squash and stir to coat in the butter and combine with the leeks. Cook until the squash is tender and floppy, about 5 minutes. Sprinkle over the thyme leaves, the salt and the pepper and stir to combine. Sprinkle over the flour and stir until there is no dry flour left visible in the pan and cook for about 2 minutes. Stir in the hot milk and bring to a bubble. Cook until the milk has thickened and created a nice sauce for the vegetables, about 5 minutes. Transfer everything to a buttered 2-quart baking dish. If you are making this ahead, leave to cool before proceeding.
- Toss the gruyere and breadcrumbs together in a small bowl, then sprinkle evenly over the top of the gratin. Cut the remaining 2 Tablespoons of butter into small pieces and dot over the top of the gratin. You can cover the cooled dish at this point and refrigerate for several hours, or bake immediately. Preheat the oven to 350 degrees and bake until bubbly and golden on the top, about 20 minutes.