Preheat the oven to 350 degrees F (175 degrees C).
Place green beans into a saucepan and fill with about 1/2 inch of water. Bring to a boil and steam until beans are tender but still bright green, about 5 minutes. Drain and toss with the mushrooms. Set aside.
Melt the butter in a large skillet over medium heat. Add the onion and parsley; cook and stir until onion is tender. Whisk in flour until blended, then mix in the lemon zest, salt, and pepper. Gradually whisk in milk, stirring constantly, so that no lumps form and then stir in sour cream. Warm over medium heat until the sauce begins to bubble. Dump in the green beans and mushrooms, stirring to coat.
Remove from the heat and toss briefly with some of the Cheddar cheese and French fried onions, saving some of each to sprinkle over the top. Spoon into a 1 1/2 quart casserole dish and top with the reserved onions and cheese.
Bake in the preheated oven until the cheese melts, about 15 minutes.