1. Before you put the roast on the rack and into the oven, add onion, celery, carrots, and garlic to the roasting pan.
2. Depending on the size of the roast, you’ll need 2 to 4 cups of homemade or low-sodium prepared stock. You can also use a mixture of stock and wine. Choose the type of stock appropriate to the roast—beef stock for beef, etc.—or try using vegetable or mushroom stock for lamb and pork.
3. Pour the stock or mixture into the roasting pan to about 1/4-inch deep.
4. Baste the roast every 15 to 20 minutes with the juices accumulating in the bottom of the roasting pan. Replenish with stock or the mixture as needed to maintain at least 1/4 inch of juices in the bottom of the roasting pan.
5. When the roast is done, remove it to a platter, tent with foil and let it rest for 15 to 20 minutes before slicing and serving.
6. Meanwhile, put the roasting pan over low to moderate heat (depending on the type of pan) and add any remaining stock or mixture. Deglaze the pan and pour through a sieve lined with cheesecloth into a large measuring cup or bowl. Press against the solids firmly with the back of a wooden spoon or potato masher to extract as much liquid as possible. Discard the solids.
7. De-fat the liquid or pour it into a gravy separator and then pour into a saucepan.
8. Put the saucepan over medium heat, bring just to a boil and then reduce the heat to a steady simmer. Taste; if you think the pan juices are not concentrated enough, raise the heat and reduce the juices to achieve a stronger flavor.
9. In a heatproof bowl, mix flour with heated pan juices (1 tablespoon of flour per 1/4 cup of stock) to make an uncooked roux that you will use to thicken the gravy. Add a tablespoon or so of the roux at a time to the pan juices whisking thoroughly; let it cook for a minute to thicken. Add more roux as necessary for a thicker gravy. Taste and adjust with salt or pepper as needed.
10. Don’t worry about a few lumps—you can run the gravy through a sieve into your gravy boat.
11. Serve hot with the sliced roast and stand back for the oohs and aahs.