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Preheat oven to 400°F. Grease a casserole dish and set it aside.
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Bring a large pot of water to a boil and add the 2 tablespoons of sea salt. Add the green beans and blanch for 6 minutes. Drain the green beans in a colander and immediately submerge the green beans into an ice bath to shock them, or stop the cooking process. Do not leave them in the ice bath for longer than a minute; drain them and set the colander aside.
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In a large skillet set over medium-high heat, cook the diced bacon until crispy and then remove the bacon from the pan. Melt the 2 tablespoons of butter in the same skillet and add the mushrooms and all of the seasonings. Cook for 4-5 minutes, stirring occasionally. Then add the garlic and cook for 1 additional minute.
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Stir the flour into the mushroom mixture and cook for 1 minute. Add the chicken broth and simmer for a few minutes. Decrease the heat to medium-low then add the half-and-half and cook until the mixture thickens, about 8-9 minutes. Stir occasionally.
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Remove the skillet from the heat and stir in the grated Parmesan cheese, green beans, bacon, and 1 cup of French (crispy) onions. Pour the green bean casserole into the prepared casserole dish. Top the casserole with the remaining French (crispy) onions and bake it for 15 minutes.
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Remove the casserole from the oven and serve immediately.