Heat a skillet over medium heat. Once the pan is hot, add the oil and diced onions and sauté for 3 minutes. Add the water and Worcestershire sauce and reduce to a simmer for 8 minutes. Remove pan from heat and set aside.
Place the ground beef between two sheets of parchment paper and use a rolling pin to flatten the beef into a rectangular sheet that is about 1/4" thick.
Remove the parchment paper and lightly season the beef with salt and pepper. Cut the beef into 12 square patties, and use a wooden spoon handle to poke a few holes in each patty.
Pour the cooked onions and juices onto a baking sheet. Place the patties on top of the onions.
Cover with another baking sheet turned upside down (this will steam the burgers). Bake at 350°F for 10 minutes.
When the burgers are done steaming, remove them from the oven. Assemble the sliders with a bottom bun, burger patty with onions, a slice of cheese, 2 pickles, and your favorite condiments.
Notes
Nutrition information is an estimate only and will vary based on exact ingredients used.