Ingredients

1. 1 pound pancetta, sliced 1/4 inch thick
2. 1 cup fresh 1/2-inch bread cubes
3. 1/3 cup whole milk
4. 2 pounds ground beef chuck
5. 3 tablespoons minced garlic
6. 1 tablespoon crushed red pepper
7. 1 tablespoon chopped thyme
8. 1 tablespoon chopped rosemary
9. Kosher salt and freshly ground pepper
10. 3 tablespoons red wine vinegar
11. 3 tablespoons water
12. 2 teaspoons sugar
13. 1/4 teaspoon cayenne pepper
14. 1 red onion, thinly sliced
15. 1 cup ketchup
16. 3 tablespoons freshly grated horseradish or drained prepared horseradish
17. Boston lettuce leaves
18. 6 crusty sourdough rolls or kaiser rolls, split and lightly toasted

Directions

1. 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.
2. 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.
3. 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.
4. 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.

Make Ahead

The uncooked burgers, ketchup and pickled onion can be refrigerated separately overnight.