Fish: Wash and dry the fish filets, cut them into bite-size pieces, place in a bowl, add the lemon juice, some salt, and white pepper. Cover and refrigerate while you prepare the rest of the ingredients.
Chop the onion and garlic cloves finely but keep them separated. Clean and chop the rhubarb into thin slices, about 1 cm/ 0.4 inch thick. Grate the turmeric finely.
Saute: Heat the oil in a large cast-iron skillet (or a non-stick pan). Cook the onions until translucent, add the garlic and stir for another minute or so. Add the rhubarb, turmeric, and curry paste and cook gently for about 3 minutes.
Cook: Add the sugar and the vegetable broth, some salt, and pepper. Stir well, then add the heavy cream. Bring to a boil and cook, uncovered, for about 5 minutes.
Mash one of the bananas and cut the other one into small cubes. Add them to the pot together with the fish and continue cooking for another 5 minutes.
Adjust the taste with more salt, pepper, and sugar if you find it necessary. Remove from heat and stir in the chopped parsley. Serve immediately with rice.