Preparation

Make the Jam

• In a heavy-duty 12-inch skillet, cook the bacon over medium heat until lightly browned but not yet crisp, about 8 minutes. Transfer the bacon to paper towels to drain. Pour off all but 2 to 3 Tbs. of bacon fat from the skillet.
• Add the onion to the skillet, season with a little salt and pepper, cover the pan, and cook for 2 minutes. Uncover, add a splash of water, and scrape up any browned bits from the bottom of the skillet. Cover, and continue to cook, stirring occasionally, until the onions are soft and beginning to color, about 10 minutes.
• Add the vinegar, mustard, and 1/3 cup water. Return the bacon to the skillet and bring the mixture to a simmer. Simmer, uncovered, until the liquid has thickened and most of it has been absorbed, 2 to 4 minutes. Transfer to a small bowl and let cool slightly. Cover with plastic wrap and leave at room temperature for up to 2 hours, or refrigerate for up to 2 days and gently reheat before serving.

Make the burgers

• With a sharp knife or food processor, mince the bacon. Transfer to a large mixing bowl, and add the ground beef, Worcestershire, 1 tsp. salt, and 1/2 tsp. pepper. Using a fork, gently toss until well mixed but not overworked. Form into 4 equal patties, each 3/4 to 1 inch thick. Press your thumb in the center of each patty to create a deep depression.

• Prepare a gas or charcoal grill fire for indirect cooking over medium-high heat (400°F to 450°F). Grill the burgers over direct heat until grill marks form, 4 to 5 minutes (move them to indirect heat if any flare-ups occur). Flip the burgers and cook for 4 to 5 minutes more for medium (a faintly pink center). For more well done, cook an additional minute.
• Transfer the burgers to a plate and tent them with aluminum foil. Toast the cut sides of the buns on the grill until golden grill marks form,about 1 minute. Serve the burgers on the buns, topped with a rounded 1/4 cup of the jam.

Make Ahead Tips

The bacon-onion jam can be made a day or two ahead, refrigerated, and gently reheated before spooning it onto the burgers.