Preheat your oven to 350°F (175°C).

Prepare an ice bath in a large bowl.

In a large pot, bring generously salted water to a boil. Add the green beans and cook for 2 to 3 minutes, stirring occasionally, until they are bright green.

Use a slotted spoon or tongs to take the beans out and place them in the ice bath to cool. Once cooled, drain the beans and transfer them to another large bowl.

In another large pot or saucepan over medium heat, melt 4 tablespoons of butter. Add the sliced onions and a big pinch of salt. Cook and stir occasionally for about 7 minutes, until they start to soften.

Add the mushrooms to the pot and cook, stirring often, until the mushrooms are browned, about 7 minutes more.

Add the finely chopped garlic and cook for another minute, stirring until fragrant. Season with salt and pepper. Transfer the onion mixture to the bowl with the green beans.

In the same pot over medium heat, melt the remaining 8 tablespoons of butter. Add the flour and cook, whisking frequently, until it turns golden and smells toasty, about 3 minutes.

Gradually whisk in the whole milk, and season with salt and pepper. Bring it to a simmer and cook, stirring occasionally, until the mixture thickens, about 4 minutes.

Pour the thickened sauce into the bowl with the green beans and stir everything together.

Transfer the mixture to a 13 by 9 inch baking dish. Bake in the preheated oven for about 30 minutes, until it is warmed through and bubbling.

Top the casserole with fried onions and continue to bake for an additional 5 minutes, until the onions are crispy and warmed through.