Heat the butter in a saucepan and sauté the onion until golden. Add the rice, toast, add the hot vegetable stock and bring to a simmer. Let the rice soak for 10 minutes over low heat, stirring frequently. Add vegetable stock if necessary.
Add the spinach (I mostly use frozen leaf spinach) to the risotto and simmer for another 15 minutes over low heat, stirring again and again so that the spinach thaws evenly.
Season the risotto with salt, pepper and nutmeg.
In the meantime, heat the oil in a pan. Pangasius fillet with salt and pepper, lightly flour and fry on both sides over medium heat for about 5 minutes until golden brown.
Mix the sour cream with the grated lemon zest and serve with the pangasius fillet and risotto.