Bring a pan of stock to the boil and add the pasta. Stir often to prevent sticking.
At about 8 minutes, tip in the grated Gruyere and the mascarpone. Stir to incorporate. Add the parmesan and stir again. You'll think it's too saucy at this stage, but the pasta soaks it up, I promise! Now cook, gently, until the pasta is al dente. It will take about 10 minutes.
Add the torn up mozzarella last of all and stir to melt it in. Don't season with salt if using a commercial stock as they are very salty anyway. Pepper is fine.
Eat as is or add extras and place under a hot grill!