You can’t have a good sauce if you start with a bad stock. Too many people take stocks for granted. In many restaurants the stock pot is like a garbage can; they throw in all kinds of trimmings. They slather the bones with tomato paste and roast them until they burn.
The ideology of a stock is important. The idea is to remove through extended gentle heat the flavor and gelatin of the bones and meat while continually removing the impurities: the blood, fat, bone, and vegetable particles released in the cooking process. You need to cook your bones and meat in the proper amount of liquid, adding the proper amount of vegetables. I like sweet things in my stock: carrot, onion, tomato, and also some garlic. I don’t use celery, because it makes the stock a little bitter. Every step of the way, you remove impurities. Everything follows from this.