Combine flour, baking powder, and salt; stir well. Cut in shortening until mixture resembles coarse meal. Sprinkle 3/4 cup milk evenly over flour mixture, stirring until dry ingredients are moistened.
Turn dough out onto a lightly floured surface; knead 10 to 12 times. Roll dough to a 12- x 8- inch rectangle.
Combine ham, 1/2 cup milk, onion, green pepper, and catsup in a large mixing bowl; stir well. Spread mixture evenly over dough. Roll up jellyroll fashion, beginning at long side; moisten edges with water to seal. Cut roll into twenty-four 1/4-inch slices; place slices, cut side down, on greased baking sheets. Bake at 425° for 15 minutes or until rolls are lightly browned.