Preheat oven to 400 F. Spray an 11 x 7-inch baking dish with cooking spray & set aside.
In a saucepan, scramble fry Italian sausage meat until cooked. Drain on paper towels & wipe out saucepan.
Add milk, half & half, Boursin cheese, garlic powder & smoked paprika. Heat over medium heat, stirring constantly until the mixture is smooth & the cheese has melted. Remove from heat & set aside.
Arrange half the sliced potatoes in overlapping rows in the prepared baking dish. Sprinkle with cooked sausage meat & pour half of the cream mixture evenly over potatoes. Repeat with the remaining potatoes, sausage & cream mixture.
Bake, uncovered, until potatoes are tender & the top is nicely browned for about 55-60 minutes. Remove casserole from oven & sprinkle with chives.