Preheat oven to 400°F. Grease a casserole dish and set it aside.
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.
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.
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.
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.
Remove the casserole from the oven and serve immediately.