Bring a large pan of water to the boil and cook the pasta following the packet instructions. Drain, reserving 100ml (3 1⁄2fl oz) pasta water.
Return the pan to the hob and add the flaked mackerel, soft cheese, onions and lemon juice, stirring until the cheese has melted. Add the pasta, reserved water and rocket and toss to combine. Serve immediately with freshly ground black pepper, if you like.