Trim the chicken pieces of excess fat, if desired. Place the chicken pieces in the bottom of the slow cooker (4qt. or larger). Mince the garlic and add it to the slow cooker. Sprinkle the oregano, cumin, red pepper flakes, and some freshly cracked pepper over the chicken.
Slice the onion, green bell pepper, and red bell pepper into strips. Place the onion and bell peppers on top of the chicken in the slow cooker. Add the can of diced tomatoes (with juices) and tomato paste to the slow cooker. The tomato paste can be added in dollops and will be stirred in after cooking.
Place the lid on the slow cooker and cook on high for four hours. After four hours, remove the lid and use tongs to stir the contents. Break the chicken into shreds as you stir the slow cooker. If the juices in the slow cooker are too thin, allow the pot to continue simmering on high without the lid for about 15 minutes to reduce the liquid. Taste and season with salt as needed (I added 1/2 tsp).
Serve the ropa in a bowl, over rice, or in a warm tortilla.