Melt the butter in a frying pan, add the chopped leeks and simmer on low heat under the lid for about 3-4 minutes. Then add the flour and mix. Pour in the milk in which the fish was cooked (gradually, half a cup), stirring gently. Cook without a lid on low heat for about 10 minutes, if necessary, add more milk or water if the sauce has become too thick.
Add broccoli, divided into small florets, capers (and peeled shrimp, if we use them). Season the contents of the pan with sea salt, freshly ground black pepper and lemon juice. Stir gently, remove from heat a minute after cooking