Line a 10x14 inch rimmed baking sheet with plastic wrap. Mix chuck , and beef stock in a large bowl. Transfer mixture to lined baking sheet and use a spatula to flatten. Place second sheet of wrap over meat, and use rolling pin or hands to further flatten meat so it covers entire surface of the pan.
Remove top layer of plastic wrap, and sue spatula to score flattened beef into 2.5 inch squares. Reapply wrap and place pan in freezer until meat is frozen solid.
Place dried onion in a small bowl,add the hot water and let stand for at least 10 minutes. Meanwhile, take frozen beef from freezer and remove the plastic wrap. Re-scroe it with a sharp knife and gently break patties apart.
Heat a large skillet,cast iron works best, or grill pan over medium-high heat. Add 1/4 of onion and water mixture to skillet. When water begins to steam place5 patties on top of onions. Cook, flipping once to desired doneness. Cooking time will depend on whether your patties are till frozen solid or have thawed a bit. After flipping, place a bun top on each patty.
Remove patties and buns, with a helping of onion from skillet. Add bun bottoms and any cheese or condiments as desired. Repeat with remaining ingredients.