Season the John Dory fillets with salt and freshly ground black pepper, then dredge the fillets in the flour on each side.
Heat the oil a frying pan until hot. Add the fish, skin-side down and cook on each side for 1-2 minutes, until just cooked through. Remove from the pan and set aside to rest in a warm place.
For the shrimp sauce vierge, heat the olive oil in a separate frying pan over a very low heat. When the oil is warm (but not too hot), add the shallots and garlic and warm through for one minute.
Add the tomatoes and shrimps and warm through for a further two minutes.
Add the tarragon, lemon juice and season, to taste, with salt and freshly ground black pepper.
For the sautéed potatoes, heat a separate frying pan until hot then add the rapeseed oil and potatoes. Sauté for 5-6 minutes, until golden brown all over and tender. Remove with a slotted spoon and drain the potatoes onto kitchen paper. Season, to taste, with salt and sprinkle with the parsley.
To serve, place the John Dory onto the centre of the plate, then spoon over the shrimp sauce vierge. Place a spoonful of the sautéed potatoes alongside.