Pancetta-Beef Burgers with Horseradish Ketchup

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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

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.