Turkey Burgers with Spinach Feta Tzatziki

Name: Turkey Burgers with Spinach, Feta & Tzatziki
Ingredients: 1 cup frozen chopped spinach, thawed; 1 pound 93% lean ground turkey; ½ cup crumbled feta cheese; ½ teaspoon garlic powder; ½ teaspoon dried oregano; ¼ teaspoon salt; ¼ teaspoon ground pepper; 4 small hamburger buns, preferably whole-wheat, split; 4 tablespoons tzatziki; 12 slices cucumber; 8 thick rings red onion (about 1/4-inch)