3 slices crust-less white bread, torn into small pieces
9 tbsp. unsalted butter, melted
Kosher salt, to taste
8 oz. hollow pasta, preferably elbow macaroni
1⁄4 cup flour
3 cups milk
4 cups grated graviera or kefalotyri cheese (about 12 oz.)
½ cup Greek Yogurt Tzatziki
3⁄4 tsp. ground cinnamon
1⁄8 tsp. freshly grated nutmeg
Freshly ground black pepper, to taste
2 tbsp. extra-virgin olive oil
8 large shallots, finely chopped
16 oz. baby spinach, roughly chopped
8 scallions cut into 1⁄4″-thick rounds
1⁄3 cup roughly chopped fresh dill
1 3⁄4 cups crumbled feta (about 8 oz.)
Put bread into the bowl of a food processor and pulse until finely ground. Put bread crumbs and 3 tbsp. butter into a small bowl and combine; set aside. Bring a 6-qt. pot of salted water to a boil. Add pasta and cook until cooked halfway through, about 3 minutes. Drain pasta, rinse with cold water, and set aside.
Heat remaining butter in a 4-qt. saucepan over medium heat. Add flour and cook, whisking constantly, for 1 minute. Still whisking constantly, slowly drizzle in milk and cook until sauce has thickened and coats the back of a spoon, 10–15 minutes. Remove pan from heat. Stir in graviera or kefalotyri cheese, Greek Yogurt Tzatziki, cinnamon, and nutmeg and season with salt and pepper; set béchamel sauce aside.
Heat oven to 350°. Heat oil in a 5-qt. pot over medium heat. Add shallots and cook, stirring often, until soft, 3–4 minutes. Add spinach and scallions and cook, covered, stirring occasionally, until wilted, about 3 minutes. Stir in the reserved béchamel sauce, the dill, and the reserved pasta and transfer mixture to a 9″ x 13″ baking dish. Sprinkle evenly with reserved bread crumbs and the feta. Bake until golden brown and bubbly, about 30 minutes. Let cool for 10 minutes before serving.