Wash the baby potatoes; halve any that are larger than bite size.
Cut fully thawed sirloin into even 1-inch pieces.
Toss the steak and potatoes with the Cajun seasoning until evenly coated.
Divide the mixture among four large sheets of heavy-duty aluminum foil. Fold each into a sealed packet with a little room for steam.
Arrange the packets in a single layer in the slow cooker and cover. Cook on LOW for 6 to 7 hours or HIGH for 3 to 4 hours, until the potatoes are fork-tender and the steak is cooked through.
Check the steak with a food thermometer; whole-cut beef should reach at least 145°F (63°C), followed by a 3-minute rest.
Open the packets away from your face to avoid hot steam and serve immediately.