Creamed Spinach Leek Casserole

In large pot of boiling water, blanch spinach, in batches, until wilted, about 20 seconds. With slotted spoon, transfer to bowl of ice water. Drain and press out water; chop finely and set aside.

In saucepan, melt butter over medium heat; cook 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 spinach, 3/4 cup (175 mL) of the cheese and mustard; cook, stirring, until hot.

Scrape into 8-inch (2 L) square glass baking dish or gratin dish. Sprinkle with bread crumbs and remaining cheese. (Make-ahead: Let cool; cover with foil and refrigerate for up to 24 hours. Bake for about 15 minutes longer.) Bake in 375°F (190°C) oven until top is crisp and golden, about 45 minutes.