Heat oven to 350 F.

In a saucepan, add the green beans and fill with half an inch of water. Bring water to a boil and cook green beans for 5 minutes until tender and bright green. Drain and toss with mushrooms.

In a large skillet, melt the butter over medium heat. Add the yellow onions and parsley and cook until the onions are tender, 3 to 4 minutes.

Whisk in flour until blended, then mix in lemon zest, salt and a pinch of black pepper.

Gradually pour in milk, stirring constantly to prevent lumps, and then whisk in sour cream.

Once the sauce begins to bubble, stir in the mushrooms and green beans.

Remove skillet from heat and mix in half the cheese and fried onions. Spoon the mixture into a casserole dish and garnish with the remaining shredded cheese and fried onions.

Bake for 15 minutes until the cheese is melted. Serve immediately.