Prepare Tomatillo Sauce; set aside.

In a medium saucepan, bring 4 cups water to a gentle simmer. Add small chopped onion, chicken breasts, and salt (with additional water to cover if necessary), skimming off any foam that rises during the first minute. Reduce heat to medium. Add herbs, partially cover saucepan, and simmer 15 minutes, or until chicken is tender. Remove from heat; allow chicken to cool in its broth. When the chicken is cool, shred with a fork and set aside. Reserve ⅔ cup of the broth.

In a large nonstick skillet, heat oil over medium-high heat, then add diced onions, garlic, scallions, and chiles and saute, stirring frequently, until the onion is lightly browned, about 5 minutes. Reduce heat to medium, add tomatoes, and cook, stirring frequently, 4 minutes. Slowly add reserved broth to tomato mixture, stirring constantly; then allow mixture to simmer until liquid is almost evaporated. Remove tomato mixture from heat. Add chicken; mix thoroughly. Season to taste. To serve, place flour tortillas in a zip-top plastic bag and microwave on high 1 minute. Place platters of tortillas, shredded chicken, and sauce on the table. Let diners top the chicken with Tomatillo Sauce, then assemble the wraps themselves.

In a medium saucepan, cover tomatillos and chiles with water. Bring to a boil over high heat, then reduce heat and simmer until tomatillos are tender, about 15 minutes. Remove from heat and drain. In a food processor, finely chop onion and garlic. Add drained tomatillos and chiles; process until smooth.

Heat oil in a medium saucepan over medium-low heat. Add tomatillo mixture and oregano. Cook, stirring constantly, until mixture thickens, about 10 minutes. Stir in chicken broth and simmer, stirring occasionally, until sauce is thick enough to coat the back of a spoon, about 20 minutes. Season with salt to taste; cool to room temperature. (The sauce can be stored in an airtight container in the refrigerator up to 2 days or frozen up to 1 month.) Bring to room temperature before serving.