This section is from the book "Cooking For Profit", by Jessup Whitehead. Also available from Amazon: Cooking for Profit.
The brown sauce which in systematic cooking we find so useful, so indispensable, even, is not much unlike the frying-pan gravy that Mary Jane makes very nicely, sometimes, by taking out the fried pork, sausage or chicken and pouring in water or milk and thickening it when it boils, but we are strictly careful to get rid of all the grease. We think over the matter an hour or two ahead of the time for making gravy to see what can be put in the pan to make it richer and to improve the color, and we make it in the roast meat pans, and generally in the oven. The material for making the gravy is the essence of beef or other meat that escapes from the meat in roasting, as already mentioned at Nos. 170, 185, 171,144 and other places, and settles at the bottom of the pan, and of course the more meat the better the gravy will be. It is well enough, but not strictly necessary to put a piece of turnip, carrot and celery in the pan along with any rough pieces of meat besides the roast, and there must be some salt put in at the beginning. All the time the meat is roasting there is more or less water in the pan and the grease and gravy are mixed together, but when the meat is taken out the pan dries down, the essence sticks on the bottom and turns brown like the outside of roast meat and the hot grease above it is as clear as water and can be poured off into ajar to be used for frying and other purposes. That being done put into the pan a quart, more or less of water or soup stock, let it boil up and dissolve the brown glaze, then add flour thickening a little at a time, making it as thick as cream, let boil and strain it into a saucepan. It is then ready for use; but if allowed to simmer at the side of the range, it will throw up scum and grease which must be skimmed off, and the sauce becomes bright and is much improved.
 
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