Preheat oven to 350 F. Spray a 9 x 13" pan with cooking spray. Layer the bottom of the pan with hash brown patties, overlapping as necessary. I also break some of the patties into pieces to fill the holes.
In a large skillet, brown the ground beef. Add the onion and cook til the onion pieces are soft. Add 1/2 cup water and the brown gravy mix. Stir until thickened.
Pour the ground beef mixture over the hash brown patties. Sprinkle the peas on top. Mix together the soup and the milk. Pour that over the peas. Sprinkle the cheddar cheese over top.
Cover and bake for 45 minutes. Uncover and add the French-fried onions. Bake another 15 minutes, uncovered.