Preheat oven to 350 degrees Farenheit. Lightly coat a 9x13 baking dish with cooking spray.
Place chicken breasts in a large pot and cover with water. Bring to a boil over high heat. Reduce heat to medium and simmer until no longer pink in the center, about 15 minutes. Drain and let cool slightly. Shred chicken by pulling apart with 2 forks; set aside. (I use my food processor).
Lightly coat a large skillet with cooking spray and place over medium-high heat. Add onion, cilantro and chiles; saute for 2 mintues. Add shredded chicken and 1 can of enchilada sauce. Cook stirring occasionally, until heated through, about 5 minutes.
Pour the 2 remaining cans of enchilada sauce in a medium bowl and microwave until warm, about 2 minutes. Dip each tortilla in the heated sauce and fill with about 1/8 of the chicken mixture. Roll up and place, seam side down, in the prepared baking dish.
Pour remaining heated sauce over enchiladas and sprinkle with cheese. Bake until enchiladas are heated through and cheese is melted, about 15 minutes.
Divide lettuce onto four plates and place a portion of enchiladas on top. Top with a spoonful of salsa, a dollop of sour cream and tomatoes. Serve and enjoy.