Wash the potatoes with a brush, put in boiling salted water - it should cover the potatoes by 2/3. Close the lid tightly and cook until absolutely soft, about 25 minutes.
Meanwhile, combine basil leaves, crushed garlic clove, sea salt and olive oil and beat until smooth.
When the potatoes are cooked, drain the water, dry them in a saucepan over low heat and mash a little with a masher. Pour over 3/4 of the basil dressing and let sit 7-10 minutes.
Thinly slice the onion into feathers. Divide lettuce mixture among bowls, drizzle with remaining dressing, top with mashed potatoes, sprinkle with onions and capers, and serve.