In a pot of cold water over high heat, boil the potatoes in their skins until just cooked, drain, then set aside to cool.
In a separate pot of water over high heat, boil the diced carrots until just cooked, drain, and place in a salad bowl.
While the potatoes and carrots are boiling, hard-boil an egg, then transfer it to a bowl of cold water.
To blanch the peas and green beans, bring a small pot of water to the boil and once it’s rapidly boiling, drop the peas and beans into the pot for 1 ½ minutes until just cooked. Use a fine mesh strainer or colander to quickly drain them, then transfer them to a bowl of iced water.
Peel and dice the potatoes and add those to the bowl; drain the peas and beans, dry them, and add those; drain the tuna and toss that in also, as well as the chopped olives, briny capers and pickled onions.
Peel and chop the egg, and fold that in with the mayonnaise, add salt and white pepper, and gently combine everything. Taste and adjust seasoning as needed, then serve immediately or refrigerate until you’re ready to eat.