Grate the cheese; set aside.
Put a large covered pot of salted water on top the stove, and get it started to boil.
Wash the zucchini, grate it skin on, and set aside in a colander to drain.
Wash the spinach leaves. If they are large, tear them up into somewhat smaller pieces. Set aside.
Add the pasta to the pot of boiling water and let it start cooking. You want to cook it until it is just barely tender to the fork.
Put butter in a microwave-proof bowl and zap in the microwave until the butter is melted. Stir in the flour well, then whisk in the milk a little at a time. Return to the microwave for anywhere from 3 to 4 minutes, removing to stir every minute, until the sauce is thick. Stir in half the grated cheese, season to taste with salt, pepper and any dried herbs that might strike your fancy (basil, oregano, etc.)
Turn your oven's broiler / grill on and get it heating.
The pasta should be ready to drain by now. Drain it, return it to the pot, and add to it the sauce, the zucchini, the spinach and the tuna. Stir to mix everything in, then turn into a shallow, ungreased ovenproof dish and sprinkle the remaining grated cheese over top.
Place under broiler / grill and leave there until everything is piping hot and the top is starting to turn golden. This should take somewhere in the range of 5 to 7 minutes. Serve hot.