Brush 1 tablespoon of the oil over the tuna. Place the crushed peppercorns on a plate, then roll the tuna in the pepper until well coated. Wrap up tightly in a piece of foil. Heat a dry, heavy-based frying pan until smoking hot. Add the wrapped tuna to the pan and cook for 7 minutes, turning every minute or so to cook evenly on each side. Remove from the pan and leave to cool a little.
Whisk the remaining oil with the vinegar until well combined, then season with salt. Just before serving, unwrap the tuna and slice. Toss the rocket with the dressing and arrange on serving plates. Scatter over the tuna and sprinkle with Parmesan shavings to serve.