Create a 10x10 piece of wax paper or parchment paper and place it on a baking sheet. Place the ground beef on the paper and spread it until it's a thin, even layer about 1/4-inch thick. Using a butter knife, divide the beef into 16 equal portions to create 2-1/2 inch square patties.
Using your finger or the back of a chopstick, create 5 holes in each patty.
Freeze the patties for an hour.
Rehydrate the onions by mixing them with the water. Allow the mixture to sit for 5 to 10 minutes.
Add the onions and water to a skillet or griddle. Bring the mixture to a simmer over medium-high heat before reducing the heat to medium-low.
Add the frozen patties to the skillet and cook the beef for 2 to 3 minutes, until the edges start to brown. Add additional water if the pan looks too dry.
Flip the hamburger patties and top each one with the slider's bottom bun, cut-side down. Place the top bun cut-side down on top of the bottom bun and continue to cook the patties until they're browned through, an additional 2 to 3 minutes.
Remove the top buns and set aside. Slide a spatula underneath the patty and flip it over onto a plate so it sits on the bottom bun. Top the patty with pickles (optional), a half slice of American cheese (if using), and garnish the burger with the condiments of your choice.
Add the top bun to the burger and serve immediately.