To make the brine, fill a medium to large stockpot with 3 quarts water. Add the kosher and pink salts, granulated and brown sugars, honey, pickling spice, coriander and mustard seeds, and garlic. Bring to a boil over high heat, stirring often to fully dissolve the salt and sugar in the water. Immediately remove the pot from the heat once the brine boils.Add 3 quarts ice cold water to a 2-gallon or larger food-safe container that will fit in your refrigerator. Pour the brine into the container and place the container, uncovered, in the refrigerator until completely cool. We divided the brine evenly between two separate containers so that it would fit in the refrigerator.
Trim the fat from the brisket until the fat layer is about 1/4 inch thick.
If necessary, cut the brisket in half so that it will fit into your container(s).
Submerge the brisket in the cooled brine.Allow the brisket to brine in the refrigerator for 5 days, flipping it daily top to bottom and stirring the brine. Make sure that if any of the brisket sides are touching one another you regularly turn them away from each other to expose all of the sides to the brine.
To cook the brisket, pour 4 cups water into the bottom of a 12 by 15 inch roasting pan. Set a rack inside the pan and place the brisket on the rack, fatty side down.
To make the spice rub, mix together the coriander, pepper and paprika in a small bowl. Evenly rub 1/4 cup of the mixture onto the top of the brisket. Then flip the brisket and rub the remaining spice mixture onto the fatty side. Allow the brisket to come to room temperature, about 2 hours.
Preheat the oven to 300 degrees with a rack low enough to fit the pan holding the brisket. Tightly cover the brisket and pan with a double layer of aluminum foil.
Bake until the meat reaches an internal temperature of 200 degrees, about 1 hour per pound or 3-4 hours total.
Without trimming the fat, carve the pastrami into 1/4 inch thick slices, or cut as thin as possible without the meat falling apart. Keep tightly wrapped in aluminum foil or plastic wrap in the fridge for up to 1 week or in the freezer for up to 6 months.
SAFETY NOTE: handle the pink curing salt with care and keep it out of reach of children. It is used in pastrami and other cured meats to kill bacteria, prevent botulism and add flavor. However it is extremely toxic if ingested directly; in fact, it's colored pink to prevent people from mistaking it for regular salt. When used with care in recipes like this, it is very safe and necessary for proper flavor and food safety. That said, you should know the risks and keep the curing salt properly labeled and out of the reach of children.