Brown meat and onions together; drain. Stir in taco seasoning mix, beans, and tomato sauce. Simmer 5 minutes.
Unroll crescent rolls into triangles. Place in ungreased 10x15 jelly roll pan or casserole(I use a 9x13). Press over bottom and up sides to form crust. Brush dough with oil; sprinkle with cornmeal.
Spoon hot mixture over crust. Bake at 375 for 25-30 minutes or until crust is golden brown. Sprinkle with cheese. Return to oven until cheese is melted, about 1 to 2 minutes. Cut into squares, top with lettuce, tomatoes and taco sauce, as desired.