Heat oven to 180C/fan 160C/gas 4. Mix the oil, tomato purée and paprika together, then coat the potatoes thoroughly in it. Squash the garlic in its skin with the flat of a knife and place on a baking tray with the potatoes.
Season well with salt and pepper, then roast for 40 mins, turning halfway through, until the potatoes have crisped up and are fluffy inside. Five mins before the end of cooking, sprinkle over the lemon juice and return to the oven. Serve with the parsley scattered over.