Wash and prick potatoes with large fork.
Cook potatoes at HIGH for 20 minutes or till done, turning and rearranging once.
Scoop potato pulp into large mixer bowl; discard skins.
Add 3 tablespoons butter, egg, salt, and pepper; beat till fluffy.
Add enough milk, if necessary, to moisten.
In 8 x 8 x 2 inch baking dish, cook mushrooms and onions, covered, in 2 tablespoons butter at HIGH for 6 to 7 minutes or till onion is tender, stirring twice.
Remove from casserole.
Spread half the potato mixture in same casserole.
Add all of the mushroom-onion mixture atop.
Sprinkle with chives.
Top with remaining potato mixture.
Sprinkle with paprika.
Cook, covered, at MEDIUM HIGH (7) for 15 minutes or till heated through.