Combine all of the pickling spice ingredients in a small bowl. Store in an airtight container.
Make the brine by putting all of the brine ingredients, except the brisket and ice cubes, into a pot and bring to a boil. Once it boils, take it off the heat and let it cool completely.
Put the brisket, brine, and ice cubes into a brining vessel, and keep it in the refrigerator or on ice for 5-7 days.
Once brining is finished, take the brisket out and rinse well with cold running water.
Put the brisket in a large pot with water to cover, the halved onion, 2 Tbsp pickling spice, and bay leaf.
Bring to a boil, then reduce the heat to a simmer. Cover and cook for about 3-4 hours, adding the optional quartered cabbage during the last hour, until tender.
Notes
Plan to start this at least 5-7 days ahead of time, as that's how long it takes to brine the meat.
If you can't fit the big pot with the brisket and brine in your refrigerator, you can brine it in a 2 gallon zip top bag that is placed in a casserole dish to prevent leaks to save space. You can also brine in in a bag on ice in a cooler.