Add the green beans to a large pot of boiling, salted water for 3 to 4 minutes or until cooked and slightly firm. Immediately add them to a large container of ice water and chill until cold. Strain and set aside.
In a large frying pan or cast-iron skillet over medium heat, add in the bacon and cook until very crispy brown, about 5 minutes. Remove the crispy brown bacon and set them aside.
Add the mushrooms and onions to the pan with rendered bacon fat over medium-high heat and brown them well, which takes about 10 to 15 minutes. Stir often.
Stir in the garlic and cook for 1 minute and then sprinkle in the flour and mix until completely combined.
Pour in the chicken stock and cook over medium heat for 3 to 4 minutes or until very thick. Pour in the cream and cook until it becomes very thick like alfredo sauce.
Stir in the crispy bacon lardons, fresh thyme, season with salt and pepper, and then fold in the chilled, strained green beans.
Place the pan in the oven at 225° for 20 to 30 minutes while making the onion straws.
Add the egg mixed with milk into a medium size bowl.
In a separate medium size bowl mix together the flour with salt, pepper, and paprika.
Add the sliced onions in egg wash and mix, and then dredge them in flour until completely coated.
Fry the onions straws in a pot or cast-iron skillet of oil at 350° until crispy brown, which takes about 1 to 2 minutes. Drain the onions on a cookie sheet tray lined with paper towels.
Add the onion straws to the top of the green bean casserole and serve.
Notes
Make-Ahead: You can make this recipe up to 1 hour before serving. Cover and keep warm in the oven at 200° before serving.How to Reheat: Add the desired amount to a casserole dish and cover with foil. Bake in the oven at 350° for 15 to 20 minutes or until hot. You can also heat in the microwave until hot.Hot to Store: Cover and keep in the refrigerator for up to 5 days.If you want to make this the night before, keeping the sauce separate from the blanched green beans and keep covered in the refrigerator until ready to cook. Mix and then add to a casserole dish, cover with foil, and bake in the oven for 20 to 25 minutes or until hot. Garnish with onion straws.You can make onion straws the night before by frying and keeping on paper towels covered in plastic wrap until ready to serve the next day.If you are using canned green beans, you will not need to blanch them, simply mix once the sauce is done cooking and place in the oven at the same temperature to heat up while making the onions straws.Yes, you can use pre-made onion straws.Perfectly substitute haricot verts for regular green beans.