Step 1

Set the oven rack to the center position. Preheat to 425ºF (218ºC).

Step 2

In a large pot, bring the water to a boil over high heat. Add in 1 teaspoon of salt and stir to dissolve. Add the cut green beans to the boiling water and cook until crisp-tender, about 3 to 4 minutes.

Step 3

Drain the beans and transfer to a large bowl filled with ice water. Chill until the vegetables are cool, about 5 minutes, then drain well.

Step 4

Heat a large skillet with high sides over medium heat. Add the butter to the pan. Once melted, add the minced garlic and saute for 30 seconds. Add the sliced mushrooms, 1 teaspoon salt, and ½ teaspoon pepper. Stir to combine. Saute until the mushrooms release their moisture and the liquid evaporates, about 5 minutes.

Step 5

Add 3 tablespoons of flour, stir and cook for 1 minute. Gradually add in the chicken stock, continually stirring to combine. Bring the stock to a rapid simmer over medium-high heat, cook until slightly thickened, about 4 minutes. Reduce heat to medium, add 1 ½ cups heavy cream. Simmer until sauce thickens, occasionally stirring for about 5 minutes. Stir in ½ teaspoon salt,  ¼ teaspoon pepper, and ⅛ teaspoon nutmeg. Turn off the heat. Season to taste. Stir in the drained cooked green beans.

Step 6

Transfer the mixture to a 13x9-inch baking dish. Bake until the sauce is bubbly and the beans are hot, about 10 to 15 minutes. Remove the dish from the oven and cover it with foil to keep warm. Meanwhile, make the onion topping.

Step 7

In a medium bowl, whisk together ½ cup flour, cup cornstarch, and 1 teaspoon salt. In a separate medium bowl, whisk the eggs.

Step 8

Coat the onion rings by dipping them first in the flour mixture, then the eggs, then back into the flour. Transfer breaded onions to a medium bowl.

Step 9

In a medium saucepan or saute pan, add vegetable oil. Heat over medium heat until the oil reaches between 350 to 360ºF (176 to 182ºC). Fry the breaded onion rings in batches until both sides are a light golden brown and crispy, about 2 minutes per side. Transfer the onion rings to a paper towel-lined baking sheet to drain any excess oil. Fry just enough onions to cover the casserole.

Step 10

Right before serving, place the fried onions on top of the hot green bean casserole. Serve immediately.