All I can say is…you won’t have any leftover of this gratin!
Preheat oven to 350 degrees F.
Add potatoes to a wide pot. Cover fully with milk, add salt to taste and bring to a boil. Reduce to low heat and simmer, uncovered, for about 20 to 25 minutes or until potatoes are fork tender. Drain, reserving the liquid. Set aside.
Meanwhile in a dry skillet, toast mustard and cumin seeds over medium-high heat until fragrant. Remove seeds from skillet and crush in mortar and pestle. Set aside. Return skillet to medium heat and add the clarified butter. Add onion, toasted seeds, garam masala, coriander and cayenne pepper and saute for 5 minutes, until onion is soft.
Butter a 12-cup shallow baking dish. Spread half the potato slices in bottom of dish. Cover the potatoes with some of the reserved liquid from cooking the potatoes. Sprinkle with half the onion mixture and salt to taste. Add remaining potatoes. Cover the potatoes with a little more of the reserved liquid from cooking the potatoes. Sprinkle with the remaining onion mixture and salt to taste. Whisk together cream and yogurt. Pour over potatoes and press down firmly to completely cover.
Bake in middle of oven for 20 to 25 minutes or until potatoes are golden and liquid has mostly evaporated. Let stand for 10 minutes before serving.