Preheat oven to 400°F. Melt 3 tablespoons butter in medium skillet over medium-high heat. Mix in 2 tablespoons mustard, then breadcrumbs. Sauté until breadcrumbs are golden, about 5 minutes. Cool briefly. Mix in cheese.
Toss 1 1/2 bags spinach in large nonstick pot over high heat until wilted, about 3 minutes. Transfer to sieve set over bowl. Repeat with remaining spinach. Press on spinach to drain.
Melt remaining 2 tablespoons butter in same pot over medium-high heat. Add leek; sauté 4 minutes. Add cream, remaining 1 1/2 tablespoons mustard, and spinach. Toss until thick and blended, about 2 minutes. Season with salt and pepper. Transfer to 7x11-inch baking dish. Top with breadcrumb mixture. Bake until bubbling, about 10 minutes.
