Mix all ingredients paste/marinade ingredients together. It should be thick like a milkshake, rub and massage paste onto the brisket, wrap tightly in foil, place in a baking pan and refrigerate for up to 24 hours.
Preheat oven to 300 degrees F. Over a medium high heat, add 1 tablespoon olive oil in a large Dutch oven and heat until oil shimmers. Sear brisket on all sides, slightly longer on the sides with fat. (If you are using a crockpot, place brisket in crockpot and add cooking stock and sliced onions, season with salt and more paprika . Cover and cook on low for 8 hours.)
If cooking in Dutch oven, add sliced onions and cooking stock, season with more kosher salt and paprika. Cover and cook for 3+ hours in the middle of the oven. Check and taste then cook for another 2 or more hours.