Preheat the oven to 200°C, 400°F, gas 6.
Whisk together the oil, vinegar and sugar with salt and pepper and taste. You want a balance of sweet and sour so you may want a touch more vinegar or sugar. Toss the pumpkin slices in the dressing in a roasting tin and roast for 20 minutes.
Remove from the oven and turn the pieces so they caramelise evenly, then return to the oven for another 15 minutes. If they look like they are burning, cover with foil. When cooked, sprinkle with mint and feta if using and serve.