Put the mushroom cubes or stock base into a glass measuring cup with 1/2 cup of water and microwave until the mushroom base has dissolved into the water, about a minute. Stir to combine.
Trim the roast, removing most of the fat even if you have to cut the roast into sections to get the fat off. (If you like the flavor of beef fat you can trim less vigorously; I prefer it to be well trimmed.)
Mix the steak rub, onion powder, salt, and black pepper. Rub all sides of the meat generously with the spice mixture.
Heat the oil in a non-stick or cast-iron frying pan and brown the meat well on all sides. This will take about 5-7 minutes; donít rush the browning step. When the meat is nicely browned, put it into the slow cooker.
Pour the mushroom stock mixture into the frying pan and cook on high for 1-2 minutes, scraping with the turner to remove any browned bits from the bottom of the pan. Pour the liquid over the meat in the slow cooker.
Cook on HIGH for 3 hours.
Wash mushrooms and cut into thick slices. Put mushrooms over the meat in the slow cooker and cook 45-60 minutes more, or until mushrooms are cooked and the meat feels tender when you pierce it with a fork.
Scoop out the meat and mushrooms with a slotted spoon; put the mushrooms in a bowl and the meat on a cutting board.
Strain the juice and taste to see if you want to simmer it to reduce and intensify the flavor. (Mine was very flavorful without doing that. You might want to use the slow cooker to keep the meat and mushrooms warm if youíre reducing the au jus or making gravy.)
Slice the meat, arrange on a platter, and serve with mushrooms and mushroom au jus. (If you donít care about low-carb you can thicken the au jus with some cornstarch and make gravy.)