You have to keep hamburgers hot for long periods of time when serving them buffet-style, and not just for visual appeal. Hamburgers have to stay out of the "danger zone," or the no man's land between 40 and 140 degrees Fahrenheit, where bacteria thrive. Keeping burgers hot is easy -- you could do it in an oven -- but you have to maintain quality, too. That's where steam trays help. Steam trays have two parts -- a water tray and an insert that steam rises through -- that not only keep burgers above 140 F, but moist and tender, too.
Set up the heat source that will keep the water in the bottom section of the steam tray hot.
If serving the hamburgers indoors, heat the oven to the warm setting. If serving the hamburgers outside, set up the grill with indirect heat -- one side lit to cook the burgers, the other side unlit to hold the steam tray.
Bring a pot of water or beef stock to a boil on the stove and turn it off. Ladle or pour 2 or 3 inches of hot water or stock in the bottom portion of the steam tray.
Place the portion of the steam tray filled with water in the oven if serving the hamburgers inside. Carefully place the water on the indirect-heat side of the grill if serving the hamburgers outside.
Place the perforated insert in the bottom part of the steam tray -- the part filled with water -- and cover it with the lid. Cook the hamburgers to a minimum internal temperature of 165 F and place them in the steam tray. Arrange the burgers in an even layer as you add them. You can keep two or three layers of burgers in the steam tray safely.
Cover the steam tray with the lid. Check the internal temperatures of the burgers every 30 minutes or so to make sure they have a minimum internal temperature of 140 F. If they measure below 140 F by more than a few degrees, raise the heat in the oven or grill as needed to get them there.
Check the water level when you check the hamburger temperature, and add more boiling water as needed to prevent the tray from frying out.
You can also pour 1 or 2 inches of hot water in an electric roasting oven and place a wire rack in it. Place the hamburgers on top of the wire rack, cover with the lid and check the water level once an hour.