In a large bowl, combine the spinach, Worcestershire, egg, and salt and pepper, to taste. With a whisk, mix the ingredients until egg is broken up and incorporated. Add the meat and with your hands, mix until thoroughly combined. Shape the meat mixture into 4 even patties, to fit the size of the kaiser roll, and put a 1/4-inch slight dimple in the middle of each patty to allow it to cook evenly.
Heat the oil in a large skillet over medium-high heat. Alternately, heat a grill to medium (350 degrees F) and rub the grate with a towel dipped in oil.
Heat the broiler on low heat.
Add the patties to the skillet, and cook for about 4 to 6 minutes. Flip the burgers and cook another 4 to 6 minutes for medium-rare burgers. Alternately, grill the patties uncovered for 4 to 6 minutes, then flip and grill for another 4 to 6 minutes. Top each patty with a piece of cheese and put under the broiler to melt the cheese. Alternately, close the grill top, and cook until the cheese is melted.
For the each burger, spread the bottom of the roll with 1/4 of the mayonnaise. Put the burger on the mayonnaise and top with 2 slices of the bacon and some of the onions. Cover with the top of the roll and serve.
Combine all the ingredients, except the onions and bay leaf, in a medium saucepan over medium heat. Stir until the sugar is dissolved, and then bring to a boil. When the mixture comes to a boil, stir in the onion. Remove the pan from the heat and add the bay leaf. Let sit at room temperature until cool, at least 1 hour.
Combine all the ingredients in a small bowl until evenly mixed. Cover and refrigerate until ready to use.