Ingredients

• 1 large red onion
• 2 pounds ground beef
• 3/4 cup dried tomato and arugula pesto, or to taste
• 6 ounces mozzarella, cut into 6 slices
• olive oil for brushing onion
• 6 hamburger buns, grilled lightly if desired
• 1 1/2 cups packed arugula, washed well and spun dry
• 1/3 cup drained bottled dried tomatoes packed in oil
• 1/4 cup olive oil
• 3 tablespoons freshly grated Parmesan
• 2 tablespoons pine nuts, toasted golden and cooled
• 1 large garlic clove, chopped and mashed to a paste with 1/2 teaspoon salt
• a pinch sugar

Instructions

1. Prepare grill.
2. Cut six 1/4-inch-thick slices from center of onion and reserve. Divide beef into 6 portions. Form indentation in each portion and spoon rounded teaspoon pesto into each indentation. Form beef portions into six 3/4-inch-thick patties, enclosing pesto completely. Grill patties on an oiled rack set 5 to 6 inches over glowing coals 5 minutes. Turn burgers and grill 3 minutes more. Top burgers with mozzarella and grill 2 minutes more, or until just cooked through. Transfer burgers to a platter and let stand while grilling onion. Brush onion on both sides with oil and grill until softened and browned, about 3 minutes on each side.
3. Spread pesto on both sides of buns and make sandwiches with onion and burgers.
4. To make the pesto: In a food processor blend together all ingredients with salt and pepper to taste until smooth. Pesto keeps, surface covered with plastic wrap, chilled, 1 week. Makes about 1 cup.
5. In a food processor blend together all ingredients with salt and pepper to taste until smooth. Pesto keeps, surface covered with plastic wrap, chilled, 1 week. Makes about 1 cup.