Good spaghetti sauces..... Start with a decent olive oil and put your diced onions in the skillet over med heat.. sautee said onions until they are transparent and caramelize.
In a crock pot transfer the onion's to the pot. Add one large can of crushed tomatoes and 2 cans of diced tomatoes. Add 2-3 bay leaves and salt/pepper to taste. I add ~ 1/4 cup of sugar as well... let this cook in the crock pot on low for about 4/5 hours....
I then brown some hamburger and add it but basically that sauce is your general purpose red sauce....