To make the mash. Bring the stock to the boil in a saucepan and boil the new potatoes for 10 minutes (or until just cooked).
Meanwhile mix the olive oil with the paprika and pepper and coat the tuna with it. Lightly coat a frying pan with oil and place the tuna on this. Cook in the pan for 4-5 minutes medium rare or 7-8 minutes for more well cooked, turning occasionally.
Then add the frozen peas to the potatoes and boil for a further 5 minutes until most of the stock has evaporated.
Remove from the heat, drain any excess stock and roughly mash with a potato masher to retain the texture then season with lemon juice, salt and pepper.
Serve the tuna on a bed of mash with the watercress on the side