Cook the pasta in 3-4 l boiling salted water (approx. 1 teaspoon salt per litre) according to the instructions on the packet. Grease casserole dish. Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer).
Clean and wash spring onions and cut into thin rings. Peel garlic and chop finely. Grate mountain cheese, dice mozzarella. Whisk milk and eggs. Season well with salt and pepper. Stir in spring onions, garlic and 2⁄3 cheese.
Wash and halve the tomatoes.
Drain the pasta, rinse with cold water and put it into the casserole dish. Pour the egg-milk mixture over it. Spread the tomatoes on top. Sprinkle with rest of cheese. Bake in a hot oven for about 35 minutes. Wash the chives and cut into fine rolls.
Remove the casserole and let it rest for about 5 minutes. Sprinkle with chives and serve.