Combine mustard, garlic, thyme, cinnamon, a pinch of salt and ½ tsp black pepper, in a bowl. Place panko crumbs in another bowl.
Working in batches, coat chicken thighs on all sides with mustard mixture, then coat completely with bread crumbs. Arrange in a single layer in a large, shallow baking pan. Dot with butter.
Bake until crust is deep golden brown and crispy, about 2 hours. Serve hot or at room temperature.
Recipe by Sprung At Last at https://sprungatlast.com/2015/01/laurie-colwins-mustard-baked-chicken/