In a pressure cooker, add banana stem, ginger, cumin, green chili, bay leaf, and 3 cups of water. Pressure cook for 3-4 whistles.
Strain the stock and set aside. Remove bay leaf, ginger, and chili from the mixture, then grind it to a smooth juice. Strain again.
In a pan, heat butter, add shallots, and sauté for 30 seconds. Add the stock and juice.
Season with salt, pepper, palm jaggery, and lemon juice. Boil gently.
Adjust seasoning, garnish with coriander, and serve warm with lemon wedges.