MELT butter in large skillet on medium-high heat. Add chicken and onions; cook until chicken is cooked through, stirring frequently.
STIR in taco seasoning mix, water and tomato sauce. Bring to boil. Reduce heat to low; cover. Simmer 5 min. Add rice and beans; mix well. Cover. Remove from heat. Let stand 5 min.
FILL taco shells evenly with the chicken mixture; top with cheese, lettuce and tomatoes.