Cut the large potatoes into halves or quarters. Bring a large saucepan of water to a boil and add the potatoes. Bring back to a boil, turn down the heat and cook for 15 minutes or until tender. Add the leek to the pan for the last 5 minutes. Drain the vegetables, then spread them out on a plate and leave them to cool. Preheat the oven to 425ºF (220ºC).
When the potatoes are cool, peel and place in a bowl with the leek, and mash. Add the milk, mustard, cheese, parsley and thyme, and mix well. Divide the mixture into 8 portions. Shape each into a ball, then press on a floured surface into a flat cake about 10 cm across and 3 cm thick.
Set the potato cakes on a clean plate. Brush with the beaten egg and sprinkle over half the breadcrumbs. Turn the cakes and coat the other side with egg and breadcrumbs. Transfer them to a nonstick baking sheet and bake for 15 minutes.
Meanwhile, place the tomatoes in an ovenproof dish, cut- side up. Brush them with the garlic oil and sprinkle over any remaining breadcrumbs.
Turn the potato cakes over carefully. Place the tomatoes in the oven with the potato cakes and bake for a further 10 minutes or until golden. Serve hot.
Per Serving: 401 calories, 18 g protein, 16 g total fat, 5 g saturated fat, 69 mg cholesterol, 44 g total carbohydrate, 7 g sugars, 8 g fibre, 314 mg sodium