In a bowl, combine tomatoes, red pepper, avocado, jalapeno, cheese, lime juice and 1/8 teaspoons salt; set aside.
In a large nonstick skillet, melt butter over medium-high heat; sauté onion for 3 minutes or until softened and starting to brown. Add chicken, chili powder, chipotle, oregano and 1/4 teaspoons salt; sauté for 5 minutes just until chicken is no longer pink inside.
Whisk flour into milk until smooth; gradually stir into skillet. Simmer, stirring, for 3 minutes or until bubbling and thickened. Line tortillas with lettuce and spoon filling on top along center; top with fresh salsa. Fold bottom of tortilla then fold sides to enclose filling.