If using fresh bitter melons, cut 0. 25 inch (6mm) off both ends, then slice the melon into 0. 25 inch (6mm) thick rounds.
Sprinkle 0. 5 tsp (0. 1 oz / 3g) of salt over the slices, toss, and let sit for at least 20 minutes before washing in fresh water, draining, and thoroughly patting dry.
Heat 4 tbsp (2 oz / 60ml) of ghee or oil in a 4-quart (4-liter) saucepan over moderate heat. Shallow-fry the fresh bitter melon rounds in a single layer until dark brown and crispy, then remove with a slotted spoon and drain on paper towels. (If using dried bitter melon, simply drop them in and fry for about 30 seconds until crisp).
In the same hot oil, add the cracked dal badi bits (2 oz / 60g) and fry for 30 to 90 seconds until nicely browned, then remove and drain on paper towels.
Add 1 tsp (0. 1 oz / 5g) cumin seeds, 0. 5 tsp (0. 1 oz / 2g) black mustard seeds, 0. 25 tsp (0. 05 oz / 1g) kalonji, and 0. 25 tsp (0. 05 oz / 1g) fennel seeds to the hot oil, frying until the mustard seeds crackle and turn gray.
Whisk 1 cup (8 oz / 240g) of plain yogurt into the pan, one spoonful at a time, ensuring each spoonful is fully absorbed into the hot ghee before adding the next to prevent curdling.
Stir in 0. 5 tsp (0. 1 oz / 2g) turmeric, 0. 5 tbsp (0. 2 oz / 7g) ground coriander, 0. 5 tsp (0. 1 oz / 2g) garam masala, the cubed potatoes (12 oz / 340g), and the cubed butternut squash (12 oz / 340g), and cook for about 5 minutes.
Pour in 2. 5 cups (20 oz / 600ml) of water or stock, the fresh curry leaves, the fried dal badi, and the remaining 0. 5 tbsp (0. 25 oz / 7g) of salt.
Bring the mixture to a boil, cover the saucepan tightly, reduce the heat to low, and simmer for 10 minutes.
Add 1 cup (16 oz / 455g) of broad beans to the pot and continue to simmer, covered, for another 20 minutes, or until the potatoes and squash are fork-tender.
Stir in the fried bitter melon and the spinach leaves during the last 5 minutes of cooking, then serve garnished with sprigs of fresh coriander or parsley.
Adjusting Broth Thickness: If you prefer a thicker, more stew-like consistency rather than a delicate soup, simply take the back of your spoon and mash a few of the tender potato and butternut squash cubes against the side of your saucepan before serving. This releases natural starches that will beautifully thicken the broth.