In a blender or food processor, add garlic, salt, pepper, mustard, brown sugar, oil, cumin, and cayenne pepper. Blend until smooth.
Rub mixture over brisket, including all the crevices. Wrap in plastic. Marinate in refrigerator for at least 24 hrs.
When ready to cook, make a bed in the bottom of the cooker with the onions. Place in brisket, fat-side up, on top of onions.
Pour in beer and water. Close and lock lid, set pressure cooker to "meat/stew", and adjust time to 45 minutes. Press start/stop.
When done, the brisket should reach an internal temperature of at least 145°F for medium-rare. When done, brisket will be tender.
Transfer brisket and onions to a roasting dish. Place under a preheated broiler in oven for 5-10 minutes until meat is browned and reaches desired crispiness.
Allow brisket to rest at least 10 minutes before slicing against the grain.