Clean and wash the spring onions. Cut into 20 cm lengths. Put aside the green tops which have been removed. Place the spring onions in an oiled, microwaveable dish. Blanche covered for three minutes at 600 watts. Meanwhile, clean, wash and slice the tomatoes. Cut the green tops of the spring onions into thin rings. Grate the cheese. Wrap 2 spring onions with a slice of ham in each case. Spread the tomato slices in the dish, place the spring onions on top of them. Mix the cheese and green tops together and sprinkle over. Season with salt and coarse ground pepper. Bake again for another five minutes at 600 watts.