Trim fat from meat. Cut meat in 2-inch pieces. Place chopped onion in a 4- to 5-quart slow cooker. Top with meat.
Combine the 1 can undrained stewed tomatoes, the green chiles, garlic, cumin, oregano and cayenne pepper in a medium bowl. Pour over meat in slow cooker. Cover and cook on low for 10 to 12 hours or on high for 5 to 6 hours.
Using tongs or a slotted spoon, transfer meat to a large bowl. Use two forks to pull meat apart into coarse shreds. If desired, drizzle meat with a little cooking liquid to moisten (discard remaining liquid). Cover to keep warm.
Heat oil in a large skillet over medium-high heat. Add bell peppers and onion wedges. Cook and stir for 4 to 5 minutes or until tender and starting to brown. Stir in the remaining can stewed tomatoes; heat through.
Spoon about 1/2 cup meat** and 1/2 cup pepper mixture onto one side of each warmed tortilla. Fold tortilla in half over filling. Top with sour cream, guacamole and salsa. Serve with lime wedges.