Preheat the oven to 200°C. Put the potatoes in the oven, directly on the shelf, and bake for 1¼ - 1½ hours until soft when pierced with a skewer. Alternatively, part cook in a microwave on high for 10-15 minutes, then put in the oven to finish baking, reducing the cooking time accordingly.
Make the salad by whisking together the olive oil, red wine vinegar and caster sugar together in a large bowl. Stir through the mint and season to taste with a little salt and freshly ground black pepper.
Toast the pine nuts in a dry frying pan until golden brown. Add to the salad dressing, along with the beetroot, onion and black olives, if using. Set aside for the flavours to mingle.
Just before you are ready to eat, stir the feta cheese through the salad. Open up each baked potato and fluff up the inside a little with a fork, adding a little butter if you like. Spoon the salad into each potato and serve immediately.