Combine first 5 ingredients in a medium bowl; stir in 1/3 cup mayonnaise and milk with a fork until moistened. Turn dough out onto a lightly floured surface, and knead 5 or 6 times.
Pat or roll dough to 1/4-inch thickness; cut with a 1 3/4-inch round cutter, and place on a lightly greased baking sheet.
Bake at 425° for 8 to 10 minutes or until golden brown. Cool slightly.
Spread each biscuit evenly with half of remaining 1/3 cup mayonnaise; top with a tomato slice. Spread tomato slices with remaining mayonnaise; sprinkle evenly with bacon and, if desired, sliced green onions.