Burgers with Mozzarella and Spinach-Arugula Pesto

8 ounces baby spinach leaves (about 10 cups packed)
2 garlic cloves, peeled
1/2 teaspoon (packed) finely grated lemon peel
 large pinch of dried crushed red pepper
4 cups (packed) fresh arugula leaves, divided (about 5 ounces)
3 tablespoons pine nuts
1 teaspoon fresh lemon juice
1/3 cup extra virgin olive oil
3 tablespoons (packed) freshly grated Parmesan cheese
1¾ pounds ground beef (20% fat)
1 teaspoon salt
1/2 teaspoon ground black pepper
6 hamburger buns, split horizontally
6  1/3 inch thick slices fresh mozzarella cheese (about 10 ounces)
2 large beefsteak tomatoes, cut into ¼ inch thick rounds

Pesto

Rinse spinach; drain briefly, then place in large glass bowl.
Microwave spinach, uncovered, on high just until wilted, 1 to 1½ minutes.
Drain and squeeze out as mush liquid as possible.
Combine garlic, lemon peel and crushed red pepper in processor; blend until garlic is finely chopped.
Add spinach, 2 cups (packed) arugula, pine nuts, and lemon juice; process until coarse puree forms.
With machine running, gradually add oil in thin stream and blend until almost smooth. Mix in cheese.
Transfer pesto to small bowl; season with salt. (It can be made 6 hours ahead. Cover; chill)

Burgers
Combine ground beef, 1 teaspoon salt, 1/2 teaspoon black pepper and 6 tablespoons spinach-arugula pesto in large bowl; mix lightly with fingertips or for just until incorporated.
Form meat mixture into six 3/4 inch thick patties. Place patties on platter.
(Beef patties can be made 6 hours ahead. Cover and refrigerate.)
Prepare barbecue (medium-high heat). Grill burgers top desired doneness, about 4 minutes per side for medium-rare. Grill buns, cut side down, just until lightly toasted, about 1 minute.
Build burgers with pesto, patties, mozzarella, cheese, tomatoes and arugula.
Cover with bun tops.
Makes 6