Bring a large pot of water to boil. Add salt and pasta, and cook pasta until al dente.
In the meantime, mix all other ingredients in a saucepan over medium heat, using a whisk to break up the tomato paste. I like to start with the tomato paste and a few splashes of milk to get it mixing, and then add the rest of the ingredients, whisking as I go.
Drain cooked pasta and add to sauce. It will seem very soupy but after about 5 minutes the pasta will absorb A LOT of the milk and will make a creamy sauce. Mix well, and then add in meatballs (see notes). Stir gently, without breaking up the meatballs.