Season chuck roast generously on all sides with salt and pepper.
Place roast in slow cooker and cover with sliced onions. Cook on high for 4 hours.
After 4 hours (and once the roast has released its juices), slide the onions into the juices. Then add two cans of cream of mushroom soup and mix with the juices. Do your best not to disturb the roast. Cook for another two hours on high.
Check for fall-apart fork tenderness. If necessary, cook for another hour or so. Then switch to warm setting. It's pretty difficult (within reason) to overcook it.
Avoid the urge to add the soup at the beginning. I know that makes it easier, but a few things will happen if you do that. First, the roast won't develop any color - and with food, color equals flavor. Second, adding the soup first will literally just boil the roast - which is not what you want to do.
If you want to take it to a whole new level, sear the roast in a cast iron skillet until caramelized on both sides before placing in the slow cooker.
Can I use a different size roast? Any size roast will work with this method. My general rule-of-thumb is this... One hour for each pound, plus one more hour.Is there a way to make this gluten-free? Yes, any gluten-free cream of mushroom soup should work just fine.Can I make this in an Instant Pot? I wouldn't recommend using this soup in a pressure cooker, but here's a version written for Instant Pot that is tried and true.