Stir together baking mix and buttermilk in a medium bowl just until blended; pat into bottom and up sides of a lightly greased 9-inch pieplate.
Bake at 400° for 7 minutes. Remove from oven, and sprinkle with 1/4 cup cheese.
Toss tomato slices with flour; arrange one layer of tomato slices over cheese. Sprinkle evenly with half of onion, basil, salt, and pepper. Repeat procedure with remaining tomato slices, onion, basil, salt, and pepper.
Stir together remaining 1 cup cheese and mustard-mayonnaise blend, and spread over top of pie.
Bake at 400° for 24 minutes or until lightly browned. Let stand 10 minutes before serving.
NoteFor testing purposes only, we used Dijonnaise for creamy mustard-mayonnaise blend and Bisquick for baking mix.