How to Make It
Lay the pancetta slices in a single layer on a baking sheet lined with plastic wrap and freeze until firm, about 25 minutes. Finely chop the pancetta and transfer to a food processor. Pulse until minced.
In a small bowl, soak the bread cubes in the milk for 5 minutes. In a large bowl, combine the ground chuck, garlic, crushed red pepper, thyme, rosemary and 1/2 teaspoon each of salt and pepper. Add the pancetta and bread and mix gently with your hands. Pat into 6 burgers, cover with plastic wrap and refrigerate.
In a small saucepan, combine the vinegar, water, sugar, cayenne and a pinch of salt and boil to dissolve the sugar. Remove from the heat, add the onion and let cool.
Light a grill. In a small bowl, combine the ketchup and grated horseradish and season with salt. Grill the pancetta-beef burgers over a hot fire until they are just cooked through, about 4 minutes per side. Put a lettuce leaf on the bottom half of each sourdough roll and top each with a burger. Spoon the pickled red onions onto the burgers and cover with the horseradish ketchup. Close the sandwiches and serve.