Instructions:

Preheat the oven to 350°F. Soak potato slices in cold water to remove starch (can be done a few hours ahead).

In a skillet, melt 4 tbsp butter and 4 tbsp olive oil over medium-high heat. Add onions and cook until caramelized. Sprinkle flour over onions.

Add chicken stock and milk, stirring until it thickens. Season, then stir in cheese and chives. Remove from heat once the cheese melts.
Grease a casserole dish. Drain potatoes and layer them, followed by the cheese sauce. Repeat.