Preheat oven to 400 degrees F. Grease a 9x13 inch baking dish (at least 2 inch sides) with butter or oil.
In a medium saucepan, mix together cream, half and half, Boursin cheese, and salt over low heat until melted and smooth. Do not bring to a boil. Remove from heat and whisk in parsley and garlic.
Arrange half of the potato slices in the prepared baking dish. Season the top with pepper then evenly pour half of the cheese sauce. Arrange the remaining potato slices over the top, slightly overlapping them. Top with more pepper and the remaining cheese sauce.
Bake in preheated oven until the top is golden brown and the potatoes are tender all the way through, 55-60 minutes.
Serve warm topped with chopped chives.