Position rack in upper third of oven; preheat to 450°F.
Whisk eggs, dill, salt and pepper in a medium bowl. Heat oil in a large ovenproof nonstick skillet over medium heat. Add leeks; cook, stirring, until softened, about 4 minutes. Add spinach and ham; cook, stirring, until the spinach is wilted, about 1 minute.
Pour the egg mixture over the vegetables and cook, lifting the edges of the frittata so uncooked egg can flow underneath, until the bottom is light golden, 2 to 3 minutes. Sprinkle cheese on top, transfer the pan to the oven and bake until the eggs are set, 6 to 8 minutes. Let rest for about 3 minutes before serving. Serve hot or cold.
Serve in a beautiful casserole dish to finish it off!
We recommend this "Pearls" casserole dish with Pyrex insert from Beatriz Ball: