Drain the tuna and place it in a small food processor along with the mayonnaise, tomato ketchup and lemon juice. Blend until smooth, then stir in the parsley and season with salt and pepper. Alternatively, you can make this by hand just mixing the ingredients together. Spoon the pâté into individual pots.
Place the sourdough slices in a preheated griddle pan to toast or under a preheated grill. Spread with butter.
Mix together the oil and vinegar and lightly dress the rocket leaves. Toss in the capers, sun-blushed tomatoes and olives.
Serve a pot of the tuna pâté along with the warm buttered toast and rocket salad.