Preheat oven to 400 degrees F. Place the green beans in a large baking dish. Top with the caramelized onions.
Heat the cream and milk over medium-low heat until it begins to steam. SLOWLY whisk in 8 ounces of the cheddar cheese, a small handful at a time and whisking constantly while allowing each addition to melt fully, until all the cheese is melted and creamy. Pour the cream mixture over the green beans. Cover with foil and bake for 30 minutes in the middle of the oven.
Remove the casserole from the oven, uncover and sprinkle with remaining cheddar cheese, place bake in the oven, uncovered, for 10 minutes more or until the cheese the melted. Let stand for 5 minutes before serving.
Notes: Prep time includes time to caramelize the onions. These can be made ahead of time, cutting prep time to ten minutes.
Pot Pies: I served the 10 ounce pot pies, which are ready in 45-48 minutes. The family size pot pies will take longer to bake.