Melt butter in a large pot, add onions and cook until translucent, about 5 minutes. Add parsley, bokchoy and carrots, and cook for another 5 minutes. Add garlic and potatoes and continue cooking for a few more minutes. Add corn, and pour in the stock, salt and pepper and bring to a boil. Lower heat and simmer for about 20 minutes (until potatoes are soft). Add fish and cook about 10 more minutes, until fish is flaky.