Soak kokum in warm water for 1 hour. Heat oil in a pan and add mustard seeds and curry leaves. Add the chopped onions and sauté for a minute then add the ginger garlic paste and cook for a minute. Add baby onions, slit green chilies. Add kokum water and chopped kokum, cherry tomatoes, and vegetable stock. Add sautéed fish with vindaloo glaze and slow simmer for 10 minutes (without a lid), or till firm to touch.