Heat oven to 375°F. Lightly spray large cookie sheet with cooking spray. In 10-inch nonstick skillet, heat oil over medium heat until hot. Add chicken and onion; sprinkle with 1 rounded teaspoon of the taco seasoning mix and mix thoroughly. Cook 5 to 7 minutes, stirring frequently, until chicken is no longer pink in center. Remove from heat.
In medium bowl, mix ranch dressing and 2 tablespoons of the taco seasoning mix until well blended. Place 1 cup dressing mixture in small serving bowl; set aside. To remaining salad dressing mixture in medium bowl, stir in Cheddar cheese, tomato, cilantro, garlic and chicken mixture.
Separate dough into 8 biscuits. Place 4 biscuits on sprayed cookie sheet; with fingers, press each into 5-inch round. Place rounded 1/2 cup chicken mixture on center of each biscuit. Press remaining 4 biscuits into 5-inch rounds; place over topped biscuits on cookie sheet, making sure top and bottom edges match up. Press edges firmly to seal.
Lift edges of filled biscuits off cookie sheet and with scissors, snip edges 1/4 inch deep and 1/2 inch apart all the way around to make a fringed look. In small bowl, mix Parmesan cheese and 1/8 teaspoon of the taco seasoning mix. Rub cheese mixture over tops of biscuits.
Bake at 375°F. for 16 to 22 minutes or until deep golden brown. Cool on cookie sheet 3 to 5 minutes. Meanwhile, place serving bowl of dressing mixture in center of large platter; arrange shredded lettuce around bowl.
Remove sandwiches from cookie sheet; cut each in half. Arrange sandwich halves on lettuce around bowl of sauce.