Heat oven to 375'. Grease 9x13" pan. Line pan with flour tortillas, bring- ing the edges of the tortillas up even with the edges of the pan. Cook ground beef and onion in 10" skillet, stirring frequently until beef is brown; drain. Stir in taco mix (dry) and tomato sauce. Spoon beef mixture into pan. Sprinkle with cheese. Beat eggs, milk, and bisquick on high speed of blender, or with wire whisk for 1 minute, until smooth. Pour over beef mixture. Bake 35-40 minutes or until tested done. Cool 10 minutes before serving. Serve with chopped tomato, shredded lettuce and sour cream if desired.