Trim excess fat from beef and cut into 2" pieces. Combine with onion in a 3-4 quart slow cooker. Mix together fajita seasoning mix (either purchased or your own homemade) and undrained tomatoes. Pour over beef. Place peppers on top. Cover crockpot and cook on low for 7-9 hours, or until beef is tender.
Combine flour with water in a small bowl and stir well to mix. Add gradually to crockpot and stir well. Cover slow cooker and cook on high for 15-20 minutes until thickened, stirring occasionally. Serve stew with hot cooked rice.
