PREHEAT oven to 400°F. Cook and stir shallots in 1 Tbsp. of the margarine in medium saucepan on medium heat until tender. Add half-and-half, mustard and remaining 1 Tbsp. margarine; cook 5 min., stirring frequently. Reduce heat to low. Add 1/2 cup of the cheese and the basil; cook until cheese is completely melted, stirring frequently.
LAYER potatoes and cream sauce alternately in greased 2-qt. round casserole dish; sprinkle with remaining 1/2 cup cheese. Cover with foil.
BAKE 45 to 55 min. or until potatoes are tender.