Heat oven to 375 degrees and butter 9x13" baking dish.
Cook macaroni according to package directions just until tender and drain well.
Melt 12 tablespoons of butter in a heavy-based medium saucepan over medium heat. Add flour, onion, bay leaf, thyme, and peppercorns; reduce heat to medium low and cook for 2-3 minutes, stirring constantly, to make a roux. Slowly whisk the milk into rux until smooth and blended. Raise heat to medium-high; whisk constantly until mixture boils. Cook for 3-4 minutes, stirring constantly, until thickened. Lower heat and continue simmering for about 10 minutes, stirring constantly.
Strain sauce into a large bowl, removing the onion, herbs, and peppercorns. Add salt, pepper, nutmeg, and cheddar cheese, stirring until cheese is melted. Toss pasta with cheese sauce and pour mixture into baking dish.
Melt last 2 tablespoons butter and toss with breadcrumbs. Spread the buttered curmbs over casserole. Bake until sizzling and lightly browned, about 40 minutes. Cover with foil is browns too quickly.