Moroccan Burger Salad

Name: Moroccan Burger Salad
Ingredients: 2 6-inch pitas, split and quartered; 1 pound lean ground beef sirloin; 1 red onion (1/2 grated, 1/2 sliced); 3 tablespoons extra-virgin olive oil, plus more for the grill; 2 tablespoons harissa (chile paste); Kosher salt and freshly ground pepper; Juice of 1/2 lemon; 2 beefsteak or heirloom tomatoes (about 1 pound), cut into 1-inch wedges; 2 Persian cucumbers, sliced 1/2 inch thick; 1/3 cup chopped fresh cilantro or mint (or a combination); 1 small heart romaine lettuce, torn; 1/3 cup crumbled feta cheese (about 2 ounces)