Add the beef roast to the base of the slow cooker. Season with salt, pepper and garlic powder. Add the onions , mushrooms and carrots around and on top of the roast. Toss in a little more salt and pepper over the veggies.
Set slow cooker on low and cook for 8 hours, or high for 6 hours. If cooking from partially frozen, you'll need to cook on low for 8 to 10 hours, depending on how frozen the pack is when you start cooking it.
To pack into gallon size freezer baggie: Label. Then, follow the same directions in the first 2 bullets. Remove as much air as you can and seal. Freeze up to 6 months in your fridge freezer or 12 months in a deep freezer.
Tothaw, place the bags in the fridge overnight. Or quick thaw in a bowl of warm water.