Cauliflower Gratin with Mustard-Sage Cornbread Crumbs

Preheat oven to 375°F. Melt 2 tablespoons butter in heavy large pot over medium-high heat. Add shallots; sauté until beginning to soften, about 4 minutes. Add cauliflower. Sprinkle with salt and pepper; toss to coat. Add broth. Cover and steam until cauliflower is just tender, 8 to 10 minutes. Using slotted spoon, transfer cauliflower to bowl. Add cream, 1 teaspoon mustard, 1 teaspoon sage, flour, and lemon peel to pot. Boil until sauce is thick, whisking, about 1 minute. Season with salt and pepper. Toss in cauliflower. Arrange cauliflower, stem side down, with sauce in 11 x 7 x 2-inch glass baking dish. Melt 2 tablespoons butter in medium skillet over medium heat. Whisk in 1 1/2 tablespoons mustard and 1 tablespoon sage. Add cornbread crumbs; toss to coat. Spoon crumbs over cauliflower. Bake until topping is golden, 20 to 25 minutes.