Start by heating a skillet over medium heat and cooking the ground beef until fully browned. Add diced onions while cooking to build flavor. Drain any excess grease before moving to the next step.
Sprinkle the taco seasoning over the cooked beef and mix well. Let it simmer for a couple of minutes so the flavors blend nicely. This step ensures every bite is packed with taco flavor.
In a mixing bowl, combine cream of chicken soup, sour cream, and milk. Stir until smooth and well blended. This mixture will create the rich, creamy texture of the casserole.
Add thawed hashbrowns to the crock pot, followed by the seasoned beef. Pour in the Rotel tomatoes and the creamy mixture. Stir everything gently to combine evenly.
Sprinkle shredded cheddar cheese over the mixture and lightly stir. This allows the cheese to melt throughout the dish instead of just sitting on top.
Cover and cook on low for 4–6 hours or high for 2–3 hours. The casserole should be hot, bubbly, and fully cooked through. Stir once halfway if possible for even cooking.