4 medium portabella mushrooms, stems removed
1 medium onion, cut into 1/2-inch slices
3 tablespoons olive oil
3/4 teaspoon salt, divided
1/2 teaspoon ground black pepper, divided
1 fully ripened Avocado from Mexico, halved, pitted and sliced
2 tablespoons low-fat plain yogurt
1/2 teaspoon minced garlic
4 whole grain hamburger buns, toasted
4 roasted red peppers