Preheat a heavy-bottomed pan and cook the pancetta until crisp (pancetta cooks much faster than regular bacon, so this only takes a few minutes). Set aside on kitchen paper and leave the residual fat in the pan.
Fry the leeks for a couple of minutes before adding the garlic. Continue to cook gently and stir frequently so that they do not brown. Remove from the heat and set aside.
Season the kingklip on both sides. Add the olive oil to the pan and fry the kingklip for a few minutes on a side until slightly golden.
Add the butter to the pan and once melted, baste the fish. Add the leeks back into the pan with the lemon juice. Transfer to the oven for a few minutes to get some colour on the kingklip.
Serve with chopped crispy pancetta, some chopped parsley and wilted greens.