Preheat a grill to medium-high. In a bowl, combine 1/2 cup plain greek yogurt, 1/4 cup grated parmesan cheese, 1 tablespoon dijon mustard, the juice of half a lemon and 1 finely chopped garlic clove. Add 1 1/2 pounds skinless swordfish, cut into 16 cubes; toss to coat. In a bowl, combine 1/4 cup toasted panko, the grated peel of 1 lemon, 2 tablespoons grated parmesan cheese and 1 tablespoon extra-virgin olive oil. Cut 2 zucchini crosswise into thirds and quarter each third lengthwise; brush with olive oil. Cut 2 large lemons into 24 half-moons. Thread 2 swordfish pieces with the zucchini spears and lemon slices crosswise onto two 12-inch skewers. Repeat to make 8 kebabs. Cover and grill on a well-oiled grate, turning once, until tender, 8 to 10 minutes. Top the swordfish with the seasoned breadcrumbs.