Sift dry dough ingredients into bowl; cut in butter. Add milk, a tablespoon at a time, tossing just until flour is dampened. Gather into ball. Pat 2/3 of dough in bottom of 8-inch round cake pan. Bake at 425 degrees until lightly browned, about 15 minutes. Cool on rack and turn oven down to 325 degrees.
Meanwhile, fry bacon until crisp; drain most of bacon fat. Cook onion in remaining bacon fat until tender. Combine remaining filling ingredients with bacon and onion.
Pat remaining dough around edge of cooled pan. Pour filling into crust. Bake at 325 degrees until almost set, about 20 minutes. Let stand 10 minutes. Cut into wedges and serve.