Line colander with triple layer of cheesecloth and set in sink. Bring milk to boil in Dutch oven over medium-high heat. Whisk in buttermilk and salt, turn off heat, and let stand for 1 minute. Pour milk mixture through cheesecloth and let curds drain for 15 minutes.
Place spinach in large microwave-safe bowl, cover, and microwave until wilted, about 3 minutes. When cool enough to handle, chop enough spinach to measure 1/3 cup and set aside. Transfer remaining spinach to blender and wipe out bowl. Place mustard greens in now-empty bowl, cover, and microwave until wilted, about 4 minutes. When cool enough to handle, chop enough mustard greens to measure 1/3 cup and transfer to bowl with chopped spinach. Transfer remaining mustard greens to blender.
Return skillet to medium-high heat, stir in chopped greens and buttermilk, and bring to simmer. Reduce heat to low, cover, and cook until flavors have blended, 5 minutes. Season with salt and pepper to taste. Gently fold in cheese cubes and cook until just heated through, 1 to 2 minutes. Transfer to serving dish, sprinkle with remaining cashews and cilantro, and serve.
Serves 4 to 6. To ensure that the cheese is firm, wring it tightly in step 2 and be sure to use two plates that nestle together snugly. Use commercially produced cultured buttermilk in this recipe. We found that some locally produced buttermilks didn’t sufficiently coagulate the milk. Serve with basmati rice.