First make the parsley dressing by putting the oil, lime juice, mustard and parsley in a screw-top jar, then shake thoroughly until combined. Season to taste.
Cook the potatoes in a pot of lightly salted, boiling water for about 15 minutes until just tender. Lift out the potatoes with a slotted spoon; leave whole if they are small or slice thickly if they are larger. Toss the potatoes in the dressing and set aside to allow the potatoes to absorb the flavours.
Bring the pot of water back to a boil and add the asparagus, then reduce the heat and simmer for 4-5 minutes until tender. Remove with a slotted spoon, refresh briefly under cold water, then drain. (This cools the asparagus quickly and helps to retain the bright green colour.) Stir the asparagus into the potatoes, along with the tomatoes and olives.
Add the eggs to the pot of water and simmer for 7 minutes. Remove, run under cold water until cool enough to handle, then peel away the shells and cut the eggs into quarters.
Add the eggs, flaked trout and arugula to the potatoes and toss lightly to coat evenly in the dressing. Serve slightly warm or cold.
Variations: The smoked trout can be replaced by canned tuna or salmon. Quail's eggs make a pretty alternative if they're available, use 4 quail's eggs to replace the 2 hen's eggs and boil for 2-3 minutes. Cut in half.
Each serving: 24 g protein, 15.5 g fat of which 3 g saturates, 66.5 g carbohydrate, 8.5 g fibre, 527 Calories