1. In a skillet cook bacon and onion until bacon is slightly crisp and onion is tender. Drain off and discard fat.
2. In a bowl stir together flour, baking powder, sugar, salt, and cream of tartar. Using a pastry blender, cut in butter until mixture resembles coarse crumbs. Make a well in center of flour mixture. Combine milk and bacon mixture; add all at once to flour mixture. Using a fork, stir just until moistened.
3. Turn dough out onto a lightly floured surface. Knead dough by folding and gently pressing dough for four to six strokes or just until dough holds together. Pat or lightly roll dough to 3/4-inch thickness. Cut dough with a floured 2-1/2-inch biscuit cutter, rerolling dough scraps as necessary. Place unbaked biscuits on baking sheet; freeze 1 hour. Transfer to a plastic freezer bag. Seal, label, and freeze up to 1 month. Makes 12 biscuits.
4. TO SERVE: Preheat oven to 350 degrees F. Place frozen biscuits 1 inch apart on ungreased baking sheet. Bake 25 minutes or until golden.