HOW TO MAKE BROWN SAUCE
The best brown sauce can only be made with the best possible stock, in other words forget about the commercial stock cubes and make your own flavorful and well-balanced stock instead. It has to be made so that no single flavor overwhelms the other, and all the ingredients should be balanced and blend well together. Base of the stock is made of bones or trimmed parts of meat, mirepoix (diced basic aromatic vegetables), and herbs or garlic. You will also need some kind of thickener, for example pure starch slurry (flour in water) or roux (flour in fat). The choices of all ingredients, the method of preparation (reduction, thickening, demi-glace etc.) all depend on the type of brown sauce you want to make. Here we describe the basics of brown sauce making, so you can feel more secure when you follow a certain sauce recipe that doesn't mention the fundamentals.
A basic formula for 400 mL/13.3 fl oz of brown sauce includes about 600 mL/20 fl oz of brown stock (made of roasted meat trimmings which gives the brown color), 200 g/7 oz additional roasted meat trimmings or bones, 50 g/1.7 oz browned mirepoix (e.g. onions, carrots, celery), 1 tbsp of tomato paste, and 1 tsp of starch or 30 g/1 oz of roux. For additional flavor you can use one bouquet garni or sachet d'epices, as well as red wine (see another article in HOW TO section).
To make the brown sauce, start with browning the mirepoix, tomato paste, and the meat trimmings in a hot oven (230 C/450 F) in a little olive oil. This procedure will add a richer flavor to the sauce and shouldn't be skipped. This is also one of the important points that differs an authentic brown sauce from the ones made of stock cubes. You can also do the browning on a stove, just be more careful not to burn the ingredients in the hot oil. If you use the stove though, add the tomato paste after some of the browning has been achieved and let the paste become half-dry (pince) and also - be careful not to burn it because it really burns quickly.
When the browning is done, pour in the hot stock and simmer the mixture for about 3 hours, or until a nice flavor has been well developed. From time to time, skim the greasy parts off the surface of the sauce (these are not so tasty impurities). During simmering the flavor juices and molecules are extracted from the solid ingredients into the liquid, and the liquid water is evaporated and thus reduced - in other words, the flavor becomes more concentrated and intensive. Before the end of the simmering, taste the sauce and throw in the bouquet garni and let simmer some more, until enough of the bouquet flavor has been extracted. Taste the sauce and season with salt and pepper if necessary.
Now is also the time to add the thickening agents, either the starch slurry (add and simmer until thickened) or the roux (add and simmer for about 3 minutes, or until the sauce is thickened). This step is optional and some sauces don't require it, they just need reduction. In fact, you can skip this artificial thickening altogether. The most important part in making a sauce is to reduce it to the optimal flavor. If it's not thick at this point, you don't necessarily have to thicken it with flour and risk spoiling the smooth texture.
The last step is to strain the sauce in order to clarify it. You can keep it fairly warm under a cover so the surface doesn't develop a solid skin.