Put the potatoes into a pan of slightly salted cold water, then bring to the boil. Cook for about 15 minutes, until tender.
Meanwhile, slice the red onion and scatter these into a big serving bowl.
Cut the roasted pepper into thin slices, add these to the bowl. If there is any cooking jus or oil in the jar, add some of this to the bowl for extra flavouring.
Rinse the tomatoes and add to the bowl, together with the olives. Add the oil and lemon juice/vinegar, then toss and set aside.
When the potatoes are cooked through, drain and add them immediately to the dressing. Season with salt and pepper. Taste and adjust the lemon juice.
Serve the potato salad warm or at room temperature.