1. Prepare mashed potatoes according to directions, spread thinly onto sheet pans and cool, about 60 mins. Reserve 5 cups.
2. Prepare hashbrowns according to directions, spread thinly onto sheet pans and cool, about 60 mins. Reserve 5 cups.
3. Mix together mashed, hashbrowns, cheese, green onions in a large mixing bowl. Portion into 2-oz servings, using a #16 scoop, onto parchment lined sheet pans, press lightly to create 2.5-in round and 0.5-in thick potato patties. Refrigerate until cool, about 1 hour or overnight.
4. On a 400° F well-oiled grill, cook patties until brown and crisp, about 4 min per side.