Heat olive oil in a large skillet over medium-high heat. Season all sides of the chuck roast generously with kosher salt and freshly ground pepper.
When the oil is hot, place the roast in the skillet and sear on all sides. Let it sit without moving for a minute or two on each side before flipping.
Transfer the seared roast and pan juices to the slow cooker. Sprinkle the onion soup mix over the roast and add water and beef broth.
Cook on high for 4-6 hours or on low for 8-10 hours.
Once the meat is tender and shreds easily with a fork, remove it from the slow cooker and shred it.
Serve the shredded meat on sliced crusty rolls with a slice of Swiss cheese. Toast the cheese-topped bun under the broiler until the cheese melts and the bun starts toasting.
Serve the sandwiches with the juices from the slow cooker as au jus for dipping.
Notes
You can add sautéed onions or peppers to the sandwiches for extra flavor.