Heat oil in heavy medium saucepan over medium heat.
Add onion; sauté until soft, about 4 minutes.
Reduce heat to low.
Add cream, cream cheese, Parmesan, and Gruyère.
Whisk until smooth, about 3 minutes.
Stir in nutmeg.
Season with salt and pepper.
Remove from heat.
Combine potatoes, 6 cups water and salt in large saucepan.
Bring to boil over high heat.
Reduce heat to medium and cook until potatoes are just tender when pierced with skewer, about 6 minutes.
Transfer potatoes to bowl.
Add olive oil and parsley; toss to coat.
Season to taste with salt and pepper.
(Fondue and potatoes can be prepared 1 day ahead. Cover separately and refrigerate. Whisk fondue 5 minutes over medium heat until melted and smooth. Reheat potatoes in 350°F oven 10 minutes.) Place potatoes on platter.