What can I add to a Mexican fajita kit to get a better taste?
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Not sure what is in your kit but sauteed onions and bell pepper are key in addition to whatever meat you are cooking. Fresh lime juice and cilantro if it has a sauce that is blah?
Agree with Sadassa _Ulna that lime juice and cilantro will help. If you're looking for dried spice suggestions, I'd go with a combination of cumin + oregano.
If the kit comes with a spice packet, try toasting the spices briefly in a dry skillet before adding the other stuff. In addition to adobo, cumin, and oregano, I highly recommend coriander! (I make fajitas from scratch with equal parts cumin and coriander and a little paprika...)