Mix the fajita mix with the chopped tomatoes and add to the slow cooker.
Add chopped onions, peppers and tomatoes - stir everything together.
Place the lid on the slow cooker and set off on low for 4 hours, or high for 2-3 hours.
Remove chicken breasts from the slow cooker and slice before returning to the slow cooker.
If the sauce is too runny in consistency for you, leave the lid off the slow cooker and cook on high for about 20 to 30 minutes to thicken it up, or transfer to a saucepan and simmer until the sauce has thickened and reduced.
Serve with tortilla wraps or load on top of jacket potatoes