Preheat the oven to 350°F. Bring a large stock pot full of water to a rolling boil. Add 3-4 tablespoons of kosher salt to the pot, followed by the green beans. Cook for 3-5 minutes, until the beans have softened but are still bright green, then, immediately remove them from the boiling water and transfer them to a large bowl of ice water. Drain the beans from the cold water, and add to a large skillet or casserole dish. Season with about 1.5 teaspoons of salt, and set aside
Heat a large skillet over medium heat on the stove. Melt butter, add onion with a pinch of salt, and cook until translucent, about 3-5 minutes. Then, add in garlic and cook until fragrant, about 30 seconds. Add in mushrooms with more salt and cook for about 5 minutes to soften.
Reduce heat to medium-low, and add chicken broth and heavy cream to the skillet. Simmer the soup mixture, and add in xanthan gum, stirring often. Allow the sauce to thicken and simmer for 5-10 minutes, adjusting the seasoning to taste.
Add the mushroom soup mixture, and toss with green beans to combine. Add the crispy topping to the top of the casserole, then bake, covered with aluminum foil for 25 minutes. Remove the foil, and continue to bake for another 10-15 minutes until the top is golden brown. Enjoy!
Store in an airtight container in the fridge for up to 4 days. We don't recommend freezing.