• Place 4 cups (8, if doubling) of chicken stock in a medium pot and bring to a boil. DO NOT SALT (if you have regular-sodium stock)!
• While stock is coming to a boil, grab a medium bowl and fill it with well-salted ice water.
• Drop green beans in boiling stock and cook for 3 minutes.
• Drain cooked green beans and immediately plunge into salted ice water. Give it a nice couple of stirs to cool it off and move the salt around. Keep the beans in the ice water and set aside.
• Melt butter in a large pan over medium heat. Once melted, add shallot and a pinch of salt and pepper. Cook until translucent, a couple minutes.
• Add mushrooms to shallot with a pinch of salt and pepper. Cook until mushroom have completely shrunk and let out their water, about 10 minutes (most of water from shrooms should be gone; drain excess if not).
• Next, add garlic to the mix and cook another minute. The smell should be intoxicating!
• Ok, to our shroom mix, throw in flour and cook for 2 minutes, stirring constantly.
• Add the remaining ¾ cup (1.5, if doubling) chicken stock to the pan and cook for 1 minute, stirring often.
• Pour in heavy cream and cook until sauce has thickened, about 10 minutes, stirring often.
• Turn off heat and drain green beans. Add beans to the sauce along with parmesan, nutmeg, lemon zest, and ⅓ cup onions. Give it a good stir and season to taste, if needed, with salt and pepper. Pour into casserole dish.

IF MAKING SAME DAY:

• Preheat oven to 425°F.
• If you want a creamier casserole, add a tablespoon of milk at a time and stir, until desired consistency is achieved. Taste sauce in case you need to season again!
• Top casserole with French's onions and bake until onions are golden brown, about 10-15 minutes, or until heated through. Keep an eye on the onions so they don't burn!

IF MAKING NEXT DAY:

• Allow the hot, freshly made casserole to come to room-temp, uncovered. Then, cover and store in fridge until next day.
• On the day of baking, take casserole out of the fridge (uncover) 1-2 hours before cooking, to come to room-temp.
• Preheat oven to 425°F.
• Add 4-5 tablespoons of milk to casserole (a few at a time), stir to loosen things up, until desired consistency is achieved. Taste sauce in case you need to season again!

  Top with French's onions and bake until onions are golden brown, about 10-15 minutes, or until heated through. Keep an eye on the onions so they don't burn!