4 tablespoons unsalted butter, melted, divided, plus more for greasing
1 cup grated Parmesan cheese, divided
1/4 cup plain bread crumbs
2 tablespoons all-purpose flour
1 1/2 cups milk
2 cups shredded Gruyere cheese, divided
Kosher salt, to taste
3 medium zucchini, sliced into 1/4-inch coins
2 teaspoons chopped fresh thyme
01Preheat oven to 375°F and grease a 11-inch oval baking dish with butter. Combine 1/2 cup Parmesan, bread crumbs and 2 tablespoons butter in small bowl until combined. Set aside.
02Melt remaining butter in medium saucepan over medium-high heat. Add flour and cook, stirring often until smooth, about 30 seconds. Slowly whisk in milk until fully combined. Cook, stirring often until mixture thickens, about 5 minutes. Remove from heat and whisk in 1 cup Gruyere cheese and remaining Parmesan until smooth.
03Overlap zucchini, from outer edge to middle of prepared baking dish. Top with one third of the milk mixture. Sprinkle with 1/4 cup Gruyere cheese and kosher salt. Repeat two times, ending with a layer of Gruyere cheese. Sprinkle with an even layer of bread crumb mixture. Bake until light golden brown and tender, about 25 to 30 minutes. Top with thyme. Serve and enjoy.