Set the large pot on your stove and heat a large amount of water for the pasta. The water should cover the pasta fully, so don't skimp on this. Cover the pot with the lid to minimize the time to boil.
Set up the pan or skillet and cover the bottom with olive oil. Turn the stove onto low heat so the oil warms slowly.
Meanwhile, prepare the large onion by halving it and cutting it into rings or pieces. The size of your onions pieces does not matter but do watch out for a relatively uniform size. This will help all the onions to sauté evenly.
Add the cut onions to the pan with the warm olive oil. Turn the heat to medium to low. Let the onions turn fragrant and glassy slowly turning when necessary. Watch out: If your heat is too high you will have burnt raw onions, which leave you with a bitter taste in your sauce. If you do burn your onions I suggest starting again.
Turn the onions over continuously until they begin to caramelize and turn golden and fragrant. They should have evaporated most of their water and you should smell the distinct smell of olive oil and onion after 10-15 minutes.
By this time the water in the large pot should be boiling. Add your pasta and very generously season with salt. You should add at least 3-4 teas spoons of salt. Do not skimp on the salt, because the pasta only spends a short amount of time in the water it's important that the water is properly salted. There's nothing worse than saltless pasta!
Immediately following this, add the concentrated tomato puree to the sautéed onions, they should be almost caramelized by this point. If the mixture is too thick add the least possible amount of water to make sure the mixture does not stick to your pan.
Once your pasta is al dente, that means almost ready, take the large pot with the boiling pasta and pour a swig of the pasta liquid into the sauce. Strain the rest of the pasta and set aside.
Taste your tomato sauce and add sugar to offset the acidity of the tomatoes. Adjust salt and sweetness to taste.
Serve immediately with Parmesan or nutritional yeast or roasted crushed cashews.