Heat oven to 200C/fan 180C/gas 6. Cut the potatoes into wedges. Pour the oil into a large roasting pan (no need to peel), toss in the wedges until coated all over, then season with pepper. Bake for 35-40 mins until crisp and golden.
Put the curry paste and tomato puree in a saucepan, stir well and fry for 1 min. Add the coconut milk, bring to the boil, then gently simmer for 10 mins. Can be frozen or made up to a day ahead. Season to taste and finish with a squeeze of lemon juice and a scattering of coriander, if using.