In a large skillet over medium-high heat, add the ground beef. Season with the salt, pepper, and garlic powder.
Drain any excess grease.
Stir the taco seasoning and water into the cooked beef and simmer for 2–3 minutes until slightly thickened.
Add the tomatoes and sour cream to the beef mixture.
Lightly grease the slow cooker. Add a layer of sliced potatoes to the bottom.
Spoon a layer of the taco beef mixture over the potatoes and spread evenly.
Sprinkle a layer of shredded cheddar cheese.
Repeat the layers (potatoes, beef, cheese) until all ingredients are used, finishing with cheese on top.
Cover and cook on LOW for 5–6 hours or HIGH for 3–4 hours, until the potatoes are tender when pierced with a fork.
Let sit for 10 minutes before serving to allow the casserole to set slightly.