Preheat oven to 250°F (125°C).
Mix together the salt, black pepper, cayenne, and crushed garlic, and rub all over your brisket. Allow the brisket sit out and come to room temperature.
Set the brisket on top of the onions and jalapeños with the fat side up.
Set your remaining jalapeños on top of the brisket.
Cover the pan tightly with aluminum foil and let bake for at least 4 hours.
Remove the brisket from the pan, and it should almost fall apart when you touch it.
Let it rest outside the pan for at least 20 minutes.
When ready to serve, trim the fat from the top, and slice very thin against the grain.