Line a colander with a triple layer of cheesecloth and set it in the sink. Bring the milk to a boil in a Dutch oven over medium-high heat, then immediately whisk in the buttermilk and salt; turn off the heat and let the mixture stand for 1 minute until curds form.

Pour the milk mixture through the cheesecloth and let the curds drain for 15 minutes. Pull the edges of the cloth together to form a pouch and twist firmly, squeezing out as much liquid as possible.

Place the taut, twisted pouch between two large plates and weigh down the top plate with the heavy Dutch oven. The cheesecloth has done its job once the cheese is firm and set, which should take about 45 minutes; remove the cloth and cut the cheese into half-inch pieces.

Microwave the spinach and the mustard greens in separate covered bowls until wilted, about 3 to 4 minutes per bowl. Let them cool slightly, then finely chop a small portion of each green to measure about a third of a cup; transfer the remaining wilted greens to a blender.

Melt the butter in a 12-inch skillet over medium-high heat. Add the cumin seeds, coriander, paprika, cardamom, and cinnamon and cook until fragrant—about 30 seconds—to bloom the spices.

Add the onion and salt. Cook, stirring frequently, until the onion is softened and translucent, about 3 minutes.

Stir in the garlic, ginger, and jalapeño and cook until the mixture is lightly browned and just beginning to stick to the pan.

Add the tomatoes and cook until the moisture has evaporated and the tomatoes begin to brown.

Transfer half of the onion mixture, half of the cashews, and the water to the blender with the greens. Process until the puree is completely smooth, about 1 minute.

Stir the puree, the reserved chopped greens, and the remaining buttermilk into the skillet with the rest of the onion mixture. Simmer over medium-high heat, then reduce the heat to low, cover, and cook for 5 minutes to allow the flavors to blend.

Season with salt and pepper to taste. Gently fold in the cheese cubes and cook until just heated through. Serve hot, sprinkled with the remaining cashews and fresh cilantro.