Soak the mustard seeds and poppy seeds in warm water for 20 minutes, then grind with the green chilli and a pinch of salt to a smooth paste. Set aside.
Heat the mustard oil in a kadai until it just begins to smoke, then lower the heat. Fry the bori, if using, until golden and crisp; remove and keep aside.
In the same oil, fry the bitter gourd rounds with a pinch of salt until lightly golden at the edges. Remove and set aside — frying first takes away most of its harshness.
Add the bay leaves, dried red chilli and panch phoron to the oil. When the seeds crackle, add the grated ginger and fry for a few seconds.
Add the potato, sweet potato, raw banana and drumstick with salt. Stir-fry on medium heat for 3 to 4 minutes, then add about 1.5 cups of hot water, cover and cook for 8 to 10 minutes until the vegetables are just tender.
Stir in the mustard-poppy paste and simmer 2 minutes — do not boil it hard or for long, or the mustard turns bitter.
Pour in the warm milk, add the fried bitter gourd, sugar and the fried bori. Simmer gently for 2 to 3 minutes until the gravy is thin, pale and coating the vegetables. Check the salt and sugar — the balance should be mildly bitter, mildly sweet.
Rest 5 minutes off the heat and serve warm with plain steamed rice, as the first course of the meal.