Adjust oven rack to middle position and preheat oven to 350ºF (175ºC.)

In a 12-inch cast iron or stainless-steel skillet, heat oil over medium heat until shimmering. Add onions and cumin seeds and cook, stirring often, until onions are softened and beginning to brown, about 8 minutes. Add the garlic, cayenne, turmeric, Kashmiri chile powder, smoked paprika, and salt, and cook, stirring frequently, until fragrant, about 1 minute.

Add potatoes and toss to evenly coat in spices, taking care to separate any slices that are sticking together. Add the tomatoes and stir to combine. Reduce heat to medium-low and cook, covered, stirring halfway through, until tomatoes soften and potatoes are cooked through and tender, about 15 minutes. Remove from heat; set aside.

In a small saucepan, melt butter over medium-low heat. Whisk in flour and cook, stirring, until mixture begins to bubble, 1 to 2 minutes. Whisk in milk gradually and cook, stirring often, until mixture thickens, about 3 minutes. Stir in cheese, allowing it to melt slightly, about 1 minute. Add cheese mixture to potatoes, gently tossing to evenly coat. Transfer to a 8- by 8-inch baking dish, and gently press potatoes into an even layer then cover tightly with foil.

Set baking dish on a rimmed baking sheet and bake until potatoes are tender and tomatoes are jammy, about 35 minutes. Remove foil and cook gratin until beginning to brown and sauce has thickened, 10 to 15 minutes. Turn the oven broiler on high and return gratin to the oven; cook until the top is golden brown and crisp, 5 to 7 minutes. (Watch gratin closely as broilers vary greatly in strength and can quickly burn unattended food.) Remove from oven and let rest for 10 minutes before serving.