Tear off 4 square sheets of heavy-duty aluminum foil (approx. 12x12 inches each) and lay them flat on the counter. Use larger sheets if needed to close the packets comfortably, or double regular foil.

Pat the raw beef chuck cubes dry with paper towels and divide them into 4 tight piles in the center of each foil square.

Sprinkle the dry onion soup mix evenly over the four raw beef piles.

Place 1 thick pat of unsalted butter directly on top of each seasoned beef pile.

Bring the edges of each foil sheet up around the beef, leaving a little space above the butter. Fold and crimp the seams securely at the top to form 4 sealed pouches.

Place the 4 sealed foil pouches seam-side up in a single layer inside a 5- or 6-quart oval slow cooker, with the lid fitting properly. Follow your model’s minimum fill and liquid requirements; do not use a dry insert if its instructions prohibit it. Do not pour liquid inside the pouches.

Cover with the lid and cook on LOW for 6 to 7 hours or HIGH for 3.5 to 4 hours, until the beef pulls apart easily with a fork. Check the center of the meat with a food thermometer: beef must reach at least 145°F before removal from the heat, followed by a 3-minute rest. Chuck needs further cooking beyond that minimum to become fork-tender; use both the temperature check and the tenderness test.

Lift the pouches onto plates with tongs, supporting the bottoms so they do not tear. Open the tops carefully, directing the hot steam away from you. Allow at least a 3-minute rest before eating, then spoon the beef and onion-butter juices over mashed potatoes, white rice or buttered egg noodles, if desired.