Black Bean Tostadas with Tofu

Name: Black Bean Tostadas with Tofu
Ingredients: 4 corn tortillas (6 diameter); 1 onion, halved and thinly sliced; 4 ounces lite firm tofu, patted dry and cut into 1/2 cubes; 1 medium zucchini (7 ounces), quartered lengthwise and sliced crosswise; 3 plum tomatoes, cut into 1/2 chunks; 1 cup canned no-salt-added black beans, rinsed and drained; 1 1/2 teaspoons salt-free chili-lime seasoning blend; 1/2 cup shredded reduced-fat Cheddar cheese; 1 cup shredded romaine lettuce (about 4 leaves); 1/2 Hass avocado, diced; 1/4 cup 0% plain Greek yogurt; 1/2 cup chopped fresh cilantro; 4 lime wedges