1. In a blender or food processor blend together the walnuts, butter, garlic, soured cream and seasoning.
2. Boil 2 litres (3 1/2 pints) water in a kettle and pour it into either a deep casserole dish or a 2.8 litre (5 pint) ovenproof glass bowl. Add 10 ml (2 level tsp) salt and the vegetable oil.
3. Bring the water back to a full rolling boil in the microwave oven set on HIGH. Quickly lower the tagliatelle into the water, three-quarters cover with cling film and microwave on HIGH for 5 minutes until it is partially cooked.
4. Remove the bowl from the oven and leave the tagliatelle to stand, covered, for about 2 minutes until it is cooked and firm to the bite. Drain well.
5. Place the tagliatelle in a large ovenproof serving dish and add the nut mixture. Toss gently until the tagliatelle is well coated. Microwave on HIGH for 1-2 minutes until it is hot. Sprinkle the tagliatelle with Parmesan cheese and serve it with a green salad.