Preheat the oven to 350 degrees F. Preheat the grill to high heat.
Start by forming the meat into 4 patties about 3/4-inch thick. Refrigerate until ready to grill.
For the relish: Combine 2 cups water, the vinegar, seeds, honey and salt in a medium saucepan. Bring to a simmer and cook over medium-high heat until the liquid has reduced to a creamy consistency, about 12 minutes. Heat the oil in a medium skillet over medium heat. Add the onions and cook, stirring occasionally until caramelized and golden brown, about 20 minutes. Add the veal stock and cook until the skillet is dry, about 3 minutes. Stir the onions into the mustard seed mixture and reserve until needed.
Meanwhile, lay the pancetta on a baking sheet and bake until golden brown and crispy, 12 to 15 minutes.
Sprinkle the burgers liberally with salt and pepper and grill until medium rare, 4 minutes per side. Toast the buns on the grill and place 1 ounce of the cheese on top of each burger. Close the lid until the cheese melts and becomes gooey, about 30 seconds.
Place the burgers on the buns and top each with the relish and 2 slices of pancetta. Garnish with the tomatoes and lettuce and serve.