1. Preheat the oven to 350°F. Butter an 8 by 8-inch covered baking dish.
2. In a large soup pot, combine the potatoes, cream, garlic, thyme, salt and pepper to taste. Bring to a boil, then reduce to a simmer. Cook until the cream is slightly reduced, about 5 minutes. Stir in the green beans and carefully transfer the mixture to the baking dish. Cover and bake 30 minutes.
3. Uncover and continue to bake until the cream is bubbling and the vegetables are tender, about 10 minutes longer. Increase the oven temperature to 400°F., scatter the gruyere and bacon over the top and bake until the cheese is melted and golden, 10 to 15 minutes longer. Remove the casserole from the oven and let rest for 20 minutes. Sprinkle with the crispy onions and serve.
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