Preheat your oven to 350°F.
Get a 4-6 quart dutch oven heating on medium heat.
Saute your diced onion in butter or oil until they're starting to brown, then it's time for the meat.
If the meat is really wet, pat dry with a paper towel. Season with salt and pepper.
Add oil or butter to your pan and brown the meat. You can do in batches if you want it really browned but I’m happy with it just in one go.
Add in garlic, give a stir, and then add in your liquids. Stir, scraping the bottom, to get all those good browned bits!
Add in mushrooms, tomato paste, worchestshire, mustard, salt, and pepper.
Put the lid on and tuck into the oven for 1 hour.
After an hour, take it out, stir it around, and gauge if it needs more. Depending on your meat, it may need another 30 minutes. You want it to easily pull apart with a fork. Depending on your pot, it may trap or let out a lot of moisture, so add a bit of water in if you think it doesnt look very saucy.
If you're going to eat it all right now, add in all the sour cream. If you plan to freeze some, take out half for freezing, add half the sour cream to Dutch oven and stir it around.