Preheat oven to 375 F. Butter an 8-inch baking dish.
In large sauté pan, heat 2 tablespoons of butter and olive oil over medium heat. Add in spinach, and allow to wilt. Note: Add spinach a bunch at a time and allow it to wilt before adding the next batch. Sauté until all of the liquid has evaporated. Remove wilted spinach from pan.
To hot pan, add ¼ cup of butter and let melt. Add leeks, pepper, salt and nutmeg, stirring occasionally, until softened, about 10 minutes. Sprinkle leek mixture with flour; cook, stirring, for 2 minutes. Whisk in milk; cook, stirring often, until thick enough to coat back of spoon, about 8 minutes. Add cooked spinach, 3/4 cup of the cheese and mustard; cook, stirring, until hot.
Our mixture into baking dish. Sprinkle top with bread crumbs and remaining cheese. Bake for 45 to 50 minutes until top is golden brown and bubbly.