HEAT oven to 350ºF.
COOK potatoes in boiling water in saucepan 10 min. or until tender; drain. Cool 10 min. Meanwhile, whisk half-and-half, mustard, egg and pepper in bowl.
ARRANGE 1/3 of the potatoes, slightly overlapping, in bottom of 1-1/2 qt. casserole sprayed with cooking spray. Top with 1 tsp. rosemary and 1/3 cup cheese. Repeat layers twice. Pour half-and-half mixture over potato mixture.
BAKE 40 min. or until golden brown.