Thoroughly wash the mustard leaves. Fill a large bowl or clean sink with cold water, submerge the leaves, and swish them around to release any grit. Lift the leaves out of the water (don't drain it) and repeat until the water is clear. Roughly chop the clean leaves into 2-inch strips.
Place the chopped greens into a large saucepan with 2 cups / 480 ml of water.
Cover, bring to a boil, then reduce the heat and simmer for 15-20 minutes, or until the leaves are completely tender.
Carefully transfer the cooked greens to a blender.
Add just enough of the cooking liquid (about 4 tablespoons / 60 ml / 2 fl oz) to help it blend. Puree until you have a thick, rustic paste.
Set aside.
While the greens are simmering, prepare the base.
Heat the oil in a large frying pan or skillet over medium heat.
Add the finely chopped onions and sauté until soft and translucent, about 5-7 minutes.
Finely dice about 1¾ inches of the ginger and cut the remaining small piece into thin matchsticks for garnish.
Add the diced ginger, crushed garlic, and sliced green chiles to the onions.
Cook for another minute until fragrant.
Stir in the cornmeal and cook for 5 minutes, stirring constantly to toast it lightly. This step is crucial for thickening the saag and adding a nutty flavor.
Pour the pureed mustard greens into the pan with the onion mixture.
Add the salt and butter.
Stir everything together well, turn the heat to low, and let it cook for at least 8-10 minutes, allowing the flavors to meld. The final consistency should be thick and creamy, like hummus.
Add a splash of water if it becomes too thick.
Serve hot, garnished with the ginger matchsticks. It's best enjoyed with corn roti (makki di roti) and an extra knob of butter.