Preheat oven to 450°. Slice the onions thinly, cover them with vinegar, and set aside.

Scrub the potatoes, cut into half-inch cubes, use clean dish towels to dry as much as possible, and spread in a single layer on one or more baking sheets. In a small skillet, heat enough olive oil to coat the potatoes. Pour the olive oil over the potatoes, salt them well, and stir so that each cube is oiled and salted. If you are using fennel, stir it in at this point. Roast until done, 20–30 minutes.

If you are using green beans or peas, boil them in salted water until tender; for peas this will be very briefly. Transfer immediately into an ice bath to stop the cooking and keep them bright green.

In a bowl large enough for your salad, combine the onions with some or all of the vinegar, mustard, and a bit more salt. When the potatoes are done, immediately transfer them to the salad bowl and toss them with the vinegar mixture. Taste for salt and vi negar. Let cool a bit.

Add in the herbs, peas or beans, and enough mayo to give it some creaminess, and toss again. This may be served warm or chilled.