Boil potatoes on medium heat until tender; drain. Set aside until cool. Cook macaroni until al dente. Drain, rinse with cold water and set aside.
Cheese Sauce: Melt butter in saucepan on medium heat. Stir in flour to make roux. Cook about 4 min. or until raw flour aroma is gone, stirring frequently. Gradually add milk, stirring until smooth after each addition. Whisk in mustard. Bring to boil. Simmer on low heat, uncovered, 3 min. Add cheese; stir until melted. Stir in cooked macaroni and bacon; set aside.
Crumb Topping: Combine ingredients; set aside.
Assembly: Cut potatoes horizontally in half; scoop out centers, leaving 1/4-inch-thick shells. (Reserve centers for another use.) Fill each potato half with 1.5 oz. macaroni mixture. Sprinkle with 1/4 oz. Crumb Topping. Arrange on sheet pan.
Bake in 400°F standard oven 9 to 10 min. or until filling is hot. Plate 2 pieces per serving.