Combine the All-Purpose Flour (1/2 cup), Water (1/2 cup), Egg (1), Garlic Salt (1 teaspoon), and Ground Black Pepper (1/2 teaspoon) and mix until combined.
Add enough Oil (to taste) to a skillet to cover the bottom by 1/2" (use a high heat oil preferred for frying) and heat to medium high heat.
Add the Onion (1) to the batter then transfer to the hot oil, being careful not to crowd the pan. Cook 2 minutes on one side or until crispy and brown.
Flip and cook 2 minutes on the other side. Transfer to a baking sheet lined with paper towels, and repeat until all onions are finished.
Add to a low heat oven to keep warm.
For the casserole, preheat the oven to 400 degrees F (200 degrees C).
Prepare an ice bath and set aside.
Bring a large pot of water to a boil and add Kosher Salt (1 tablespoon). Add the Green Beans (1.5 pound) and cook until bright green and crisp, 3-4 minutes.
Immediately transfer to an ice bath and allow to cool. Drain and reserve until needed.
Heat the Butter (1 tablespoon) and Olive Oil (2 tablespoon) in a large skillet over medium high heat. Add the white part of the Leek (4) and Button Mushroom (8 ounce) and cook until soft and tender, about 5 minutes. Have Dried Thyme (to taste), Ground Nutmeg (1 pinch), Sage Leaves (4) out and ready and season with a sprinkle of each.
Add the Garlic (2 clove) and cook until fragrant, about 30 seconds.
Add All-Purpose Flour (2 tablespoon) and toss to combined. Cook for one minute, then pour in the Chicken Broth (1 1/2 cup). Increase the heat and simmer to thicken, stirring occasionally.
Stir in the Heavy Cream (1/2 cup). Taste and adjust seasonings.
Stir in the green beans and a quarter of the fried onions. Transfer to a baking dish and bake for 15 minutes or until bubbly.
Top with the reserved onions and eat away!