Thanksgiving wouldn't be complete without this beloved side dish, a favorite that Siri Pinter and Carson Daly has as part of their feast.
Preheat the oven to 350 degrees F.
To parboil the green beans, bring a large pot of salted water to boil. Add green beans and cook for 2 minutes. Remove with slotted spoon and place into a big bowl of ice water to stop the cooking process. Set aside.
Heat the butter and olive oil over medium heat in a large skillet. Add the onions and mushrooms and sauté for a few minutes, until softened and fragrant. Add the flour and stir until no lumps remain. Add the chicken stock and milk (or cream) and continue to stir. Let the sauce come to a boil, then lower heat and let simmer for 10 minutes or until sauce begins to thicken, stirring frequently. At this point, I like to taste it and then season it with appropriate amounts of salt, pepper and a pinch of nutmeg.
Drain the water out of the bowl of green beans. Once the sauce is ready, add to the bowl and toss to combine. Pour green bean mixture into a casserole dish, and bake for 30 minutes.
While it's baking, heat vegetable oil in a medium saucepan over medium-high heat, enough to cover about an inch of pan. Add the shallots and let fry until they turn golden, roughly 7 to 8 minutes. Remove with slotted spoon and let cool on a paper towel-lined plate. When green bean casserole is done baking, top with crispy shallots.