Place the milk, flour and butter in a saucepan and whisk together over a gentle heat until the sauce begins to simmer and thicken.Season with salt and pepper and a good grating of nutmeg.
Turn the heat down to its lowest setting and let the sauce cook for 3 minutes, then stir in the grated Parmesan, remove from the heat, cover with a lid and leave on one side while you prepare the other ingredients.
Add the tomatoes to the pan, and let them bubble and reduce for about 1 minute. Turn the heat off.
Next cook the pasta in plenty of boiling salted water for 6 minutes only; then drain it in a colander. Return it to the saucepan, add the sauce and the asparagus mixture and mix thoroughly.
Taste to check the seasoning, pour the whole lot into the gratin dish, sprinkle with the shavings of Pecorino cheese and bake in the oven for 8-10 minutes.