Trim the tuna of any membrane or gristle. Wrap tightly in clingfilm and put in the freezer for about 1 hour until just frozen but not rock solid.
Meanwhile, whisk together the lemon juice, oil, garlic and capers in a bowl. Add salt and pepper to taste and whisk until emulsified.
Unwrap the tuna and thinly slice with a sharp, thin-bladed knife. Arrange the slices on 4 large serving plates. Spoon the dressing over the tuna. Top with a tangle of rocket leaves and scatter with Parmesan shavings.