Cut the polenta into approximately 1.5cm cubes, toss in flour and fry in a little oil until slightly coloured. Keep warm.
Heat oven to 190C/170C fan/gas 5. Heat the remaining oil in a non-stick pan and fry the salmon portions for 1 min on each side until lightly golden, then transfer to a non-stick baking tray and cook in the oven for 8-10 mins. Test the fish to see if it is cooked by pushing a cocktail stick into the top of it. If the fish offers resistance, then it is not fully cooked and will need further cooking. When cooked, remove from the oven and set to one side.
Dress the watercress with a little olive oil and a few drops of lemon juice. Scatter the polenta croutons and capers over each salmon fillet and serve with the watercress.