Place peppers in a 3 quart pot and cover with water, at least 2 inches above peppers.
Bring water to a boil, reduce heat to medium-low, and let peppers simmer for about an hour. Use a fork or spatula to gently flip peppers a few times so they cook evenly.
Remove stems from the cooked peppers. Place peppers, garlic, onion, 1/4 teaspoon of salt and about 1 cup of cooking liquid into a blender (recommended) or food processor. Blend until completely smooth. Taste and add more salt if needed.
Sauce can be used immediately or refrigerated for up to 2 days.
Heat a large skillet over medium-high heat with a tablespoon of olive oil. If using a large roast, cut into 4-5 pieces. Season with a little salt and pepper and place in the skillet, browning on each side; about 2 minutes per side.
Place meat in the slow cooker. Pour sauce over the meat and stir to coat completely.
Cook on low for 6-8 hours. You will know it's done because the meat will easily fall apart when pierced with a fork.
Shred meat in the slow cooker and re-cover. Continue to cook on low for 30 minutes.
Place some cooked white or Mexican rice in a flat bottomed bowl or on a plate. Top with shredded meat and a small amount of green cabbage. Add a few squeezes of lime juice and a pinch of salt. Serve hot.
Treat these like your favorite burrito bowls and add alllll the toppings!