Line a colander with a triple layer of cheesecloth and set in sink. Bring milk to boil in large, heavy-bottomed pot over medium-high heat. Whisk in buttermilk and salt and remove from heat. Let stand 1 minute. Pour through cheesecloth let drain 15 minutes.
Pull together edges of cheesecloth to form pouch. Twist pouch, extracting as much liquid as possible. Place pouch between two plates and set a heavy pot on the top plate. Let stand at room temperature 45 minutes, or until cheese is firm and set. Drain whey as often as necessary. (Cheese can be wrapped in plastic and refrigerated up to 3 days at this point.) Unwrap and dice cheese into 1/2" pieces.
Place spinach in large bowl and microwave on high until wilted, about 3 minutes. Let cool until it can be handled.
Chop enough spinach to make 1/2 cup and set aside. Place remaining spinach in blender container.
Wilt mustard greens in same bowl in microwave, about 4 minutes. Let it cool until it can be handled.
Chop enough mustard greens to make 1/2 cup and set aside with chopped spinach. Add remaining mustard greens to blender container.
Meanwhile, melt butter in large skillet over medium-high heat. Add cumin seeds, coriander, paprika, cardamom, and cinnamon, and cook, stirring often, until fragrant, about 30 seconds. Add onion and 3/4 tsp salt; cook until soft and translucent, about 3 minutes. Add garlic, ginger, and jalapeno, and cook, stirring often, until lightly browned, about 2-3 minutes. Add tomatoes and cook until pan is dry and tomatoes are beginning to brown, 3-4 minutes. Remove from heat and transfer half the mixture to the blender.
Add half the cashews and all the water to the blender. Process on high until smooth, about 1 minute. Add puree to skillet and return to medium-high heat. Add reserved greens and buttermilk and bring to simmer. Reduce to low, cover, and cook until flavors blend, about 5 minutes. Add salt and pepper to taste. Gently fold in cheese cubes and cook until hot, 1-2 minutes. Serve sprinkled with remaining cashews and cilantro.