Venison, even from farm-raised deer, is much leaner than conventional beef. This affects how you cook it, because fat is an insulator. The leaner, denser flesh of a deer will cook more quickly than a comparable piece of beef, and requires both a lower temperature and a shorter cooking time. To prepare a deer roast in the oven, you'll need to reduce the temperature by approximately 25 degrees Fahrenheit to avoid overcooking and toughening the venison. A deer shoulder is relatively fatty, for venison, but will also be tough unless it's slow-roasted for an extended period.
Wipe down your roast with paper towels to dry the surface and remove any stray bone fragments or other debris left by the butcher. If your deer was wild-caught, check it for shot damage as well.
Heat a heavy skillet over a medium-high burner until it's very hot. Add a tablespoon of vegetable oil and sear the roast on all sides, until it's well browned and very savory smelling.
Season the roast liberally with salt and pepper. If you wish to use additional spices, herbs or other seasonings, rub or sprinkle them on at this point. Cover the roast with thin slices of pork fat, if you wish, to protect it from drying out during the long cooking process.
Fill the bottom of a heavy roasting pan with the onions, celery and carrots, a traditional flavoring mixture known by the French name "mirepoix." Put the rack into the roaster, jiggling it as necessary to make it fit over the vegetables.
Place the roast on the rack and transfer the roasting pan to a preheated 275 F oven. Roast at this temperature until the shoulder shows an internal temperature of 135 F when tested with a thermometer. After resting, this will leave the roast rare to medium-rare inside.
Remove the roast from your oven once it reaches the desired degree of doneness, and cover it loosely with aluminum foil. Allow the meat to rest for 15 minutes before carving.