In a deep pot put the 1 liter of water and bring to a boil.
Then add salt and cooking oil to prevent the pasta from sticking together.
Put the Spaghetti or Fettuccine and cook according to the instruction on the package.
Stir the pasta from time to time to avoid the pasta sticking on the bottom of the pot.
Check if the pasta is already cooked and tender, firm but not hard (al dente).
Then drain the water from the pasta and fill the pot with tap water to prevent the pasta from overcooking.
Drain the water again and pour the pasta in a colander or strainer to remove the excess water.
Let it drain for a few minutes then transfer to a Tupperware or any container that can hold the pasta. Set aside.
In a medium size saucepan, in a medium fire, melt the butter and saute onion and garlic until tender then add the sliced mushrooms.
Stir fry the mushrooms until the sides are seared and a little brown.
Then add the tuna and stir fry for 1 to 2 minutes and season with magic sarap, salt and pepper.
Add in the milk, water, cheese, cream of mushroom soup and nestle cream and stir and simmer until the sauce boils.
Continue stirring until the sauce becomes thick. Add the parsley and stir again for a few seconds. Turn off heat.
Pour the carbonara sauce on top of the Spaghetti or Fettuccine and garnish with parsley and grated cheese. Or just toss the pasta on with the sauce and serve.