In a large bowl combine the ground beef, Worcestershire sauce and salt. Mix with your hands and divide into 6 equal pieces.
To form the patties, I like to use a ring mold just a little bigger than the bun. Place the mold on a 1/2 sheet pan and press the mixture into the mold. Repeat with the remaining 5 pieces.
Cover and refrigerate until ready to cook.
Fry the pancetta until cooked through and crispy. Place on a paper towel lined plate and reserve for later.
Place mayonnaise, Dijon mustard, 1 tablespoon Worcestershire & chopped capers into a small bowl, mix and refrigerate until ready to assemble the hamburgers.
Heat a large sauté pan over medium high heat. Add the olive oil and butter. When the butter has melted add the mushrooms. Spread the mushroom into an even layer and allow to cook undisturbed for about 4 minutes or until starting to brown and the mushroom liquid is releasing
Add the shallots, garlic, thyme and salt. Stir to combine and continue to cook until the mushroom liquid has dissipated, the mushrooms are tender and beginning to brown, about 8-10 minutes
Taste and adjust seasoning if necessary. Set aside
Remove the hamburger patties from the refrigerator
Preheat your grill to medium/high
When the grill is hot, cook the patties for about 3-4 minutes per side for medium. Add the cheese slices toward the end of the cooking time to melt the cheese.
To assemble, open the buns and place cut side up on a platter or cutting board. Smear the top and bottom with the sauce, place some arugula on the bottom bun, then top with the patty, mushrooms and 2 pieces of the crispy pancetta. Cover with the top bun. Repeat with the remaining 5 buns.