Tear off four 12-inch pieces of heavy-duty aluminum foil. Lightly spray the center of each piece with cooking spray.
In a large bowl, combine the steak, pepper and onion blend, and Cajun seasoning. Toss everything together until evenly coated.
Divide the mixture evenly among the four foil pieces, placing the ingredients in the center of each piece.
Fold the long ends of the foil over each portion, then fold the sides over to create tightly sealed packets.
Place the packets directly into the slow cooker, stacking them if necessary.
Cover and cook on LOW for 4 to 6 hours or until the steak is fork-tender.
Carefully open the foil packets, watching for hot steam, and serve warm.
