Burger with Taleggio cheese, pancetta and onion-mustard sauce

Ingredients:

Hamburger

• 900 g of ground beef (the preferred proportion of meat and fat is 80/20, respectively)
• 4 hamburger buns
• 110 g of sliced ​​Taleggio cheese
• 1 chopped Bull's Heart tomato
• Leaves from one head of Boston lettuce

Onion and mustard sauce

• 1/2 cup champagne vinegar
• 3 tablespoons mustard seeds
• 1 tbsp. honey
• 1 teaspoon of salt
• 1 tbsp. vegetable oil
• 3 medium onions, cut into strips
• 1/4 cup beef broth
• 8 very thin slices pancetta

Preparation:

1. Preheat oven to 180°C. Heat grill to high.
   
   Start by forming the meat into four patties, about 2 cm thick. Refrigerate until the grill is hot.
2. Prepare the sauceCombine 2 cups of water, champagne vinegar, mustard seeds, honey, and salt in a medium saucepan. Bring to a boil and simmer over medium-high heat for 12 minutes, until the liquid has reduced to a creamy consistency. Heat the oil in a medium skillet over medium heat.
3. Add the onion and cook, stirring occasionally, for 20 minutes, until golden brown and caramelized. Add the broth and cook for 3 minutes until completely evaporated. Add the onion to the mustard mixture and set aside until needed.
4. Meanwhile, place the pancetta on a baking sheet and bake for 12 to 15 minutes, until crisp and golden brown.
5. Sprinkle the patties generously with salt and pepper and lightly grill for 4 minutes per side. Toast the buns and top each patty with 1 ounce of cheese. Close the lid for 30 seconds, until the cheese is melted and gooey.
6. Place the patties on buns, spread each with sauce, and top with two slices of pancetta. Garnish with tomatoes and lettuce leaves and serve.