Taken from "Charcuterie by Michael Ruhlman" (I've folleowd a fair few of his recipes and had great results)
Pastrami differs from corned beef in two main ways: it’s smoked and it’s coated with a combination of crushed coriander seeds and black peppercorns. Other than that, it’s corned beef underneath a smoky crust. The coriander-peppercorn crust is a spicy flavorful seasoning that works well on most meats, as well as fish and poultry. The best way to serve pastrami is to reheat it gently for hours until it’s falling-apart tender (steamed or gently roasted, see step 7 below). We favor the beef plate, a cut from the breast or below the shoulder: because it is especially fatty, it stays juicy after all the cooking, and thus makes an especially fine sandwich. Any good butcher shop will be able to provide you with this cut.
THE BRINE
1 gallon/4 liters water
11⁄2 cups/350 grams kosher salt
1 cup/225 grams sugar
11⁄2 ounces/42 grams pink salt (8 teaspoons)
1 tablespoon/8 grams Pickling Spice (page 70)
1⁄2 packed cup/90 grams dark brown sugar
1⁄4 cup/60 milliliters honey
5 garlic cloves, minced
One 5-pound/2.25-kilogram beef plate or beef brisket, heavy surface fat removed
1 tablespoon/8 grams coriander seeds, lightly toasted (see Note page 52)
1 tablespoon/10 grams black peppercorns, lightly toasted (see Note page 52)
1. Combine all the brine ingredients in a pot large enough to hold the beef and bring to a simmer, stirring until the salt and sugar are dissolved. Remove from the heat and allow to cool to room temperature, then refrigerate until the brine is chilled.
2. Place the beef in the brine and put a plate on top of it to keep it completely submerged. Refrigerate for 3 days.
3. Remove the beef from the brine, rinse it, and dry it. Discard the brine.
4. Combine the coriander and pepper in a spice mill or coffee grinder and pulse until coarsely ground. Coat the beef evenly with the mixture.
5. Hot-smoke the beef (see page 77) to an internal temperature of 150 degrees F./65 degrees C. (Traditionally, pastrami is first cold-smoked, then hot-smoked to achieve a heavy smoke. So try to get as much smoke on it as possible by keeping it below its final temperature for as long as possible.)
6. To prepare the pastrami for serving, preheat the oven to 275 degrees F./140 degrees C.
7. Place the beef in an inch of water in a stockpot or on a rack above the same amount of water in a roasting pan. Bring the water to a simmer, then cover the pot, place it in the oven, and slow-roast or steam for 2 to 3 hours, until it’s fork-tender.