1 pound fresh green beans or haricot vert, trimmed and blanched
1 onion, chopped
1 8 ounce package button or crimini mushrooms, minced
kosher salt and black pepper to taste
2 tablespoons butter
2 tablespoons flour
2 cups whole milk
1/2 cup Parmesan cheese, grated
Preheat oven to 350F.
Butter or spray a 9x13 casserole dish. Spread blanched green beans in the casserole dish evenly, and set aside.
Heat a large skillet over medium heat. Add butter, then add mushrooms and onion. Saute vegetables about 10 minutes - the mushrooms will first release moisture, then the moisture will evaporate.
Sprinkle flour over mushrooms, stirring to combine. Slowly add milk, stirring constantly. Bring to a boil, reduce to a simmer, and allow sauce to cook for about 2-3 minutes. Remove from heat and season to taste with salt and pepper.
Pour sauce over green beans evenly. Sprinkle the top with the Parmesan cheese. Bake for about 45 minutes, or until top is golden and the beans are tender.