Preheat oven to 350 degrees F. Spread tortilla strips in an even layer on two large baking sheets. Bake 15 minutes. Cool on a wire rack.
Meanwhile, in an extra-large skillet heat oil over medium-high heat. Add onion, garlic, and oregano; cook about 3 minutes or until onion is tender, stirring occasionally. Add tomatoes and chipotle chile pepper(s) plus the 1 teaspoon adobo sauce. Bring to boiling; cook 1 minute, stirring occasionally. Add broth; return to boiling. Simmer, uncovered, 5 minutes. Stir in cooked chicken and black pepper; heat.
Set aside a few of the baked tortilla strips. Gradually stir the remaining tortilla strips into the chicken mixture; heat. Remove from heat; sprinkle with cheese. Top with cilantro sprigs and the reserved tortilla strips. In a bowl combine sour cream and milk; serve with the casserole.
