Wash the potatoes and simmer them in plenty of unsalted boiling water for about 12 minutes (dont let them become soft). You can check how much they have cooked using a fork (or a small skewer): the fork should be easy to insert without the potatoes crumbling or splitting. If you are using yellow potatoes (as opposed to new potatoes) keep them whole and cook them for about 15 to 18 minutes longer. When the potatoes are cooked, pour off the water and allow them to cool. If you are using organic new potatoes, you can eat them with their skins on. If not, remove the potato skins completely.
Slice the Tropea onions finely and marinade them in the juice of one lemon (this has the effect of making the raw onion easier to digest).
In the meantime, simmer the green beans in boiling water for about 8 minutes or until they are cooked. To check whether or not they are cooked, take a bean and hold it horizontally at one end the other end should bend downwards if theyre done. Remove the cooked green beans from the water using a slotted spoon (reserve the cooking water for the broad beans) and plunge them into iced water to halt the cooking process and to preserve their fresh green colour.
Remove the fresh broad beans from their pods and, depending on their size, cook them for 3 to 5 minutes in the same water used for the green beans. Once cooked, drain the beans and remove their skins before placing them in the iced water with the green beans.
Drain the olives; drain the capers and rinse off the salt. Quarter or halve the tomatoes and slice the potatoes. Chop the green beans into pieces.
To make the dressing, combine the extra virgin olive oil, lemon juice, dried oregano leaves, salt and pepper.
Place all of the vegetables and the beans into a large bowl, add the fresh basil leaves, and season with the oil and lemon juice dressing. Carefully combine everything together. I always recommend mixing with washed clean hands as I find its the best way to gently bring all the ingredients together.