Pour applesauce over raw beef chuck roast, together with 3 ingredients, into slow cooker for a hearty meal that’s my default when I don’t know what to make

Scatter the sliced onion evenly around and over the chuck roast so the onions will cook down into the sauce.
Sprinkle the dry onion soup mix evenly over the top of the roast and onions. This provides most of the seasoning and deepens the savory flavor.
Lightly season the top of the roast with the kosher salt. The soup mix already contains salt, so this is just to round out the flavors; you can always adjust at the end.
Pour the applesauce directly over the raw beef chuck roast, letting it flow over the top and down the sides so the meat is mostly coated and some applesauce seeps down to the onions below. This is the key step that creates the apple-rich cooking liquid and tenderizes the beef.
Cover the slow cooker with its lid. Cook on LOW for 8 to 9 hours, or on HIGH for 4 to 5 hours, until the chuck roast is very tender and easily pulls apart with a fork.