Place the steak, peppers, and onions in a slow cooker. Season with fajita seasoning and toss to coat.
Pour tomatoes over top of mixture and spread out evenly.
Cook on low for 6 hours or on high for 3-4 hours.
Once cooking time is finished, mix in the zest and juice of the limes and cilantro if using.
Serve with a slotted spoon for fajitas or with the cooking liquid over rice or quinoa.
Peppers and Onions: Use any color of peppers or a variety of peppers as the recipe states. You can also opt to use a 16-ounce bag of frozen pepper and onion strips in place of fresh peppers and onions--no need to defrost!Leftovers: Once cooled, any leftover steak fajita mixture can be stored in an airtight container for up to 4 days. You can also opt to freeze the mixture for up to 1 month.For the Fajita Seasoning: use 1 teaspoon chili powder, ½ teaspoon cumin, ½ teaspoon smoked paprika (or regular paprika), ½ teaspoon salt, ¼ teaspoon onion powder, ¼ teaspoon garlic powder, and a dash of cayenne pepper.