Bring a large pot of water to a boil and add green beans. Blanch for about 5-8 minutes or until they turn bright green and are tender. Drain.
Melt butter in a frying pan and add onions and garlic. Saute until onions are tender and then add in mushrooms. Cook for a few more minutes to soften the mushrooms.
Add in flour and stir to combine. Add in lemon zest, salt, pepper and sugar. Gradually add in milk, stirring constantly to avoid lumps.
Add in sour cream. Stir over medium heat until it bubbles and thickens. Add in green beans and parsley. Stir to coat.
Toss in half of cheese and 1/2 cup french fried onions. Reserve remaining for the top.
Place in a 11 x 13 dish and top with remaining cheese and fried onions
Cook in oven for 15 minutes or until heated through.
Notes
Tips:Canned greens can be used, just drain first. They will have a softer texture. Frozen green beans can be used without blanching them first. Thaw beans before using.Store any leftovers in an airtight container in the refrigerator for 2-3 days. The crunchy fried onions will soften up after being refrigerated.You can make this green bean casserole the night before, cover and refrigerate. Make sure to add the crispy onions right before baking so they don't get soggy sitting in the refrigerator over night.