Pierce potatoes with a fork; arrange in a circle on paper towels in a microwave oven. Microwave at HIGH 6 minutes or until done; cool slightly. Cut each potato in half lengthwise; scoop out pulp, leaving a 1/4-inch-thick shell. Transfer pulp to a medium bowl.
Add milk and butter to potatoes; beat with a mixer at medium speed until well blended. Add onion and next 3 ingredients, beating well. Arrange shells on a baking sheet and spoon potato mixture evenly into shells. Sprinkle evenly with cheese; broil 1 to 2 minutes or until cheese melts.