Place the ground beef and yellow onion in a large skillet over medium heat.
Cook until the beef is browned and the onion is soft, stirring to break up the beef as it cooks (usually about 6–8 minutes).
Drain the beef and transfer the mixture to a slow cooker.
Add the tomato paste, salt, cream of mushroom soup, sliced mushrooms, and water. Stir to combine.
Cover and cook on low for 4 hours or on high for 2 hours.
Turn off the slow cooker, and stir in the sour cream.
Serve over egg noodles or rice.
Notes
You can substitute fresh mushrooms for the canned mushrooms, or leave them out if you don't like them.Any type of ground beef (80%, 85%, or 90% lean) are fine here.I recommend using full-fat sour cream. Low-fat and no-fat versions can curdle when heated.Store any leftovers in the fridge for 4–5 days. You can also freeze the finished dish for up to 2 months.Need dinner quickly? You can make the stroganoff entirely on the stovetop. After draining the beef and onion, add them back to the skillet, and add the other ingredients (except the sour cream). Allow to simmer for 15 minutes. Take off the heat and stir in the sour cream, and serve.