Sift together flour, baking powder and salt. Cut in butter or margarine.
Beat egg and add it to ½ cup milk. Blend into the flour mixture. Divide dough into two pieces. Roll out ⅓ inch thick and cut into two 9 inch circles. (Or you can pat dough out on parchment paper sprayed lightly with Pam.
Toss together drained tuna, ¼ cup milk, onion, parsley, sweet pickle, and salt and pepper to taste.
Place one shortcake on a greased baking sheet and spread with filling. Place other shortcake on top. Bake in 425-degree oven for 20 minutes or until done.
For the sauce, simply melt Velveeta in a double boiler and gradually add a ¼ cup milk.
Here's the crowning glory for your shortcake (and for dozens of other main dishes). Place shortcake on a serving dish and cut into six wedges before pouring over the cheese sauce.