- Cook the buckwheat: Put the buckwheat and the water or stock into a large pan and add 1/2 tsp salt and ground black pepper. Bring to the boil, then cover and simmer gently for 10 minutes or until the liquid has been absorbed. Remove from the heat and allow to stand, still covered, for 5 minutes, then fork through the grains.
- Cook the vegetables: While the buckwheat is cooking, put the milk and water into a pan and add a little salt. Add the cauliflower and leek, and cook for 7 minutes or until just tender. Transfer the vegetables with a draining spoon into a large, shallow, warm ovenproof dish. Mix the buckwheat grains and spinach leaves into the vegetables.
- Make the sauce: Preheat the grill to high. Blend the cornflour with 3 tbsp cold water to make a paste and stir briskly into the hot milk mixture. Stir over a medium heat until the sauce starts to thicken. Add the butter and simmer for 1 minute, then remove from the heat and stir in two-thirds of the cheese until melted. Check the seasoning and pour over the vegetables and grains, mixing in lightly.
- Top the dish: Dot the surface with the tomato halves and scatter with the remaining cheese. Grill until the top is browned and bubbling. Serve hot.
Cooled, cooked buckwheat can also be used as the base for a salad. Stir in chopped vegetables and toss with vinaigrette.
Add some crispy pancetta rashers, or dry-fry a pack of lardons until crisp. Scatter over the vegetables before covering with the sauce.
Buckwheat can be cooked, cooled quickly and frozen for up to one month. To use, reheat in the microwave, partially covered in a bowl, for 4-5 minutes on High, forking through the grains once. To use in a salad, leave to thaw at room temperature for 2-3 hours or thaw in the microwave on High for 3-4 minutes.