For the tomatillo salsa- Line a baking sheet with aluminum foil and preheat oven to 350.
Put sliced tomatillos, onion, and poblano pepper in a bowl and stir a few times, add oregano, salt, and garlic powder and then pour in olive oil and stir so that everything is covered with olive oil. Spoon contents of the bowl onto the lined baking sheet and bake for 30-45 min until the onions are slightly browned on the outsides. Remove from oven and allow to cool for 10 minutes, then put the contents of the baking sheet (minus the aluminum foil of course) into a food processor and pulsate until desired consistency is reached (I like mine smooth and not chunky.)
For the chicken and bell pepper- In a cockpot put the chicken, sliced bell pepper and tomatillo salsa (you want the salsa to coat the chicken and peppers, you could always add a little water if you don't have enough salsa.) Cook on high for 2.5 hours, and then shred the chicken with two forks.
For the tacos- To assemble the tacos set up the taco shells on a plate, then stuff each taco with some shredded chicken and peppers, then top with a little shredded cheese and cilantro and drizzle sour cream over tacos, then enjoy!!