Preheat oven to 350 F. Butter the inside of a 3-quart baking dish.
Bring the heavy cream, garlic, bay leaves, thyme, peppercorns, mustard and nutmeg to a simmer in a small saucepan over low heat. Let cool slightly. Remove bay leaves, thyme and peppercorns from sauce. Season the cream mixture with salt and pepper.
Fan out a layer of potatoes on the bottom of the buttered casserole dish. Sprinkle with salt and pepper and top with 1 cup of the kale. Pour 1/4 of the cream mixture on top. Sprinkle with 1/4 of the grated cheese mixture. Repeat with 3 more layers, reserving the last 1/4 cup of cheese.
Bake potatoes until tender and creamy, around 1 hour. Then cover the potatoes with foil and let bake for another 30-35 minutes, or until the cream is bubbling and the potatoes are easily pierced with a fork.
Place rack in highest position; heat broiler. Mix the breadcrumbs with the remaining 1/4 cup of cheese and sprinkle over the potatoes.
Broil until cheese is bubbling and top of gratin is golden brown, 7-10 minutes. Serve topped with sprigs of fresh thyme leaves, if desired.