In a cast iron or heavy bottom skillet, heat 2 tablespoons olive oil and brown the pork on medium high heat until the sides of the meat have a caramel color. Do not overcrowd the pan; work in batches.
Remove meat from skillet and place in the bowl of a slow cooker.
Add 2 more tablespoons of olive oil and saute the onions and peppers over medium high heat for about 10 minutes or until the onions are translucent and softened.
With a slotted spoon, strain and scoop out the onions and peppers and place them with the meat in the bowl of the slow cooker.
Add the Masa, stirring to combine and cook over medium heat for about 1 minute, stirring often.
Add the garlic and cook for another minute.
Add the beer and water and whisk to smooth out any lumps.
Whisk in the tomato sauce, cumin, chili powder, salt and pepper.
Pour gravy over meat, stirring to combine.
Cover and set slow cooker to low. Cook for about 6 hours or until meat is tender.
Serve hot with warm tortillas, diced onion, chopped fresh cilantro, sliced avocado and lime wedges.