Remove roast from fridge and place on a rack for thirty minutes to come closer to room temp (skip this step if in a hurry).
Heat a cast iron skillet or sturdy Dutch oven over medium high heat about five minutes. Meanwhile, pat dry the roast so it will brown properly.
Sprinkle kosher salt over hot pan, and add roast, browning on both sides.
Meanwhile, slice onion and mushrooms. Set aside.
When roast is brown, remove to crock pot; turn on low.
Add onions and mushrooms to skillet, without wiping it first. Cook, stirring frequently, until onions are brown and mushrooms have given up their liquid. The mixture should be dry but not burnt.
Add water and stir, scraping bottom of pan to get all of the flavorful bits.
Pour hot onion mixture over roast. Cook on low eight hours (or high for four).
If you want to make gravy, drain the liquid off the roast at the end of the cooking time, into a liquid cup measure. You should have about two cups. Add water if necessary or remove excess liquid until you do have two cups. Melt two tablespoons butter in a skillet or medium saucepan. When foam subsides, add three tablespoons all purpose flour. Stir a few minutes to cook the flour, then whisk in liquid slowly to avoid lumps. Cook until thick and serve over meat and mashed potatoes or rice.