Preheat flat top grill or griddle on medium heat.
Saute diced onions until slightly tender. Season onions with salt, pepper, and garlic powder.
While onions are cooking, combine shredded hash browns and salt, pepper, and garlic powder in a large bowl.
Fold sour cream, cream of chicken soup, shredded cheese, jalapenos, and sauteed onions into the hash brown potatoes. Combine thoroughly, but don't smoosh the hash browns.
Put a heat-proof, round silicone mold on the preheated flat top grill and place a small pad of butter in the mold. Add hash brown casserole mix to the mold, being careful not to overpack the mold. Remove the mold and repeat this step until all of your casserole mix is on the grill.
Alternate method - If you don't have a mold, you can also mound up about 1 cup of hash brown casserole on the flat top grill using your spatula and form it into a patty shape, about 3/4" to 1" thick.
Allow the hash brown casserole patties to cook for about 5-7 minutes on the first side, or until nicely golden brown. Then flip and cook for an additional 5 minutes on the second side.
Optional - Sprinkle more shredded cheese and diced jalapenos on top of each patty and allow the cheese to melt.
Carefully remove the cooked hash brown casserole cakes from the flat top griddle and allow to cool slightly before serving.