6 potatoes, pared and cut into 3/4 inch cubes (about 2 1/2 lbs.)
3 tbsp. butter
3 tbsp. flour
1 c. milk
1/2 c. heavy cream
2 tsp. Dijon style mustard
1/4 tsp. salt
1/8 tsp. pepper
Dash nutmeg
1 c. shredded Gruyere cheese, divided

Grease a 1 1/2 quart shallow baking dish; set aside. Cook potatoes in boiling salted water 7 to 10 minutes. Preheat oven to 350 degrees. Meanwhile in medium saucepan, melt butter over medium heat. Add onion; cook, stirring, 4 minutes or until soft. Add flour; cook stirring, 1 minute. Gradually add milk, cream, mustard, salt, pepper and nutmeg. Cook, stirring constantly, until thickened. Remove from heat. Stir in 2/3 cup of the cheese until melted. Drain potatoes well and add to sauce; toss well. Spoon into baking dish. Sprinkle with remaining 1/3 cup cheese. Bake 20 minutes. Place under broiler and broil 2 minutes or until lightly browned.