Bring the milk, buttermilk, and salt to a simmer in a large pot over medium-high heat, stirring occasionally to prevent scorching. Cook until the mixture reaches 190 degrees, large curds form, and the surrounding liquid (the whey) is clear.

Line a colander with three layers of cheesecloth and set it over a large bowl. Pour the milk mixture into the colander and let the curds drain for 15 minutes. Gather the edges of the cheesecloth and twist tightly to wring out as much excess liquid as possible.

Place the cloth-wrapped cheese between two plates and weight the top plate with a heavy can or pot. Press the cheese until it is firm and compact, then remove from the cloth and cut into 1/2-inch cubes.

Place the spinach and mustard greens in a large microwave-safe bowl, cover with plastic wrap, and microwave until the greens are completely wilted. Let them cool slightly, then squeeze them very dry by hand and chop coarse.

Melt the butter in a large skillet over medium heat. Add the cumin seeds, coriander, paprika, cardamom, and cinnamon; stir until the spices are fragrant. Add the onion and a pinch of salt, then cook until the onion is softened and lightly browned, about 8-10 minutes.

Stir in the garlic, ginger, and jalapeño until fragrant. Add the tomatoes, a quarter-cup of the cashews, and water. Simmer the mixture until the liquid has reduced by about half.

Transfer the tomato mixture to a blender. Add the buttermilk and half of the chopped greens, then process until the sauce is completely smooth. Return the sauce to the skillet.

Fold the cubed cheese, the remaining chopped greens, and the remaining chopped cashews into the sauce. Simmer gently over medium-low heat until the cheese is heated through and the flavors have melded.

Stir in the cilantro and season with salt and pepper to taste. Serve hot with rice or warm naan.