1. Sauté chopped onion in a lightly greased skillet over medium-high heat 5 to 6 minutes or until tender. Stir in next 6 ingredients. Remove from heat. Tear corn tortillas into bite-size pieces, and stir into soup mixture. Stir in half of shredded Cheddar cheese.
2. Spread chicken mixture into 6 (2 1/2-cup) buttered oven-safe bowls or ramekins, and top evenly with remaining cheese.
3. Bake, covered, at 325º for 45 minutes or until bubbly. Uncover and bake 15 more minutes or until golden brown.
Note: Chicken Sopa may be baked in a buttered 13- x 9-inch baking dish as directed. To make ahead, assemble, cover tightly, and freeze up to 1 month. Thaw in refrigerator overnight, let stand at room temperature 30 minutes, and bake as directed.