To make the sofrito, add all ingredients to a blender or food profcessor and pulse until very finely chopped and there are no large pieces or leaves left. Reserve ¼ cup for this recipe and save the rest in ice cube trays for later use.
For the brisket, start by mixing together the sazon, salt, cumin, oregano and paprika and generously season the meat on all sides, pressing the spices into the meat so it adheres.
Heat a large cast iron skillet on high heat (my preference, but use what you have) and drizzle with oil. Once hot, sear brisket on all sides until caramelized and deep golden brown. Once done, remove and set aside.
Next, add the sliced onion and peppers to the bottom of the slow cooker, then add the seared brisket, fat side up, along with the stock, tomato paste and sofrito.
Cook on low for 6-8 hours, until the meat is incredibly tender.
Once done, remove meat from the slow cooker and let rest before cutting into it. If there is a fat cap, carefully cut excess fat off and discard
When ready to cut, slice brisket against the grain. Meaning, slice the meat the opposite direction of the muscle fibers, which will make the meat tender and easier to enjoy.
Place brisket on platter and spoon peppers, onions and any residual sauce on top.