Set out the slow cooker and tear off 4 large sheets of heavy-duty aluminum foil, each about 12 to 14 inches long; double the foil if it seems flimsy.

Pat the steak strips dry with paper towels, then place them in a large bowl.

Sprinkle the Cajun seasoning over the steak and toss with clean hands or tongs until the strips are coated all over; let them sit for 10 to 15 minutes if desired.

Lay the foil sheets flat and divide the seasoned steak evenly among them, piling each portion in the center and leaving room around the edges for sealing.

Drizzle the melted butter evenly over the steak in all four packets.

Bring the long sides of each foil sheet over the steak and fold them together several times, then fold and crimp the short ends to seal each packet completely with the seam on top.

Place the packets seam-side up in the slow cooker, stacking lightly if needed while keeping them mostly in one layer or slightly overlapping.

Cover and cook on LOW for 4 to 6 hours or on HIGH for 2 to 3 hours, using the shorter time for sirloin and the longer time for round steak.

Carefully open one packet away from your face and hands; the steak should be cooked through, easy to cut with a fork, and reach at least 145°F internally. If needed, reseal it and cook for another 30 minutes before checking again.

Turn off the slow cooker and use tongs to transfer the packets to plates or a platter. Open them at the table and spoon the buttery Cajun juices over the steak and any sides.