For the rocket salad and dressing, whisk the chopped shallots and ketchup in a bowl, then add the vinegar and Worcestershire sauce and stir to combine.
Add the olive oil little by little, whisking continuously until thickened, then stir in the chopped chervil and tarragon. Season to taste with salt and freshly ground black pepper, add the rocket leaves and toss well to combine. Set aside.
For the potatoes, heat the oil in a non-stick frying pan over a medium heat, add the partially cooked potato slices and season to taste with salt and freshly ground black pepper. Fry the potatoes for 3-4 minutes, or until golden-brown on both sides. Remove from the pan and drain on kitchen paper.
Heat a non-stick chargrill pan over a medium heat. Drizzle the tuna steaks with a little oil and season with salt and freshly ground black pepper. When the pan is hot, add the steaks and cook for 45-50 seconds on both sides until the outsides of the steak have browned but the inside is still pink, or until cooked to your liking.
To serve, spoon the sautéed potatoes into the middle of each serving plate. Cut the tuna steaks in half and place them on top of the potatoes. Pile the dressed salad alongside.