Preheat the oven and smoker. Preheat the oven to 400 degrees F. Also preheat your smoker to 275 degrees F with your favorite hard wood. I recommend pecan, apple, hickory, or alder for this recipe.
Bring a pot of water to a boil. Fill a large pot with water and stir in 1 tablespoon of salt. Bring to a boil.
Prepare your crunchy onion strings. Place your sliced onions in a gallon-sized zip top bag and pour in the gently beaten eggs. Toss to coat onions in the eggs. In a small bowl, combine 1/4 cup of the flour, the panko, and 1 teaspoon salt. Pour into a shallow pan. Transfer the egg coated onions to the panko mixture and toss gently to coat.
Bake the onions. Cover a baking sheet with a piece of aluminum foil and coat liberally with nonstick spray. Evenly distribute the onions on the foil, making sure they don't overlap too much. Bake in the high temperature oven, stirring gently every 5 minutes for a total of 20 minutes. Remove when the onions are golden brown. Set aside to cool until needed.
Blanch the green beans. Blanch the trimmed green beans by gently placing them in the salted boiling water for 5 minutes. Drain and rinse them in icy cold water.
Sautee the mushrooms. Melt the butter in a 12-inch cast-iron skillet over medium-high heat. Add the sliced mushrooms, a teaspoon of salt, and black pepper. Cook, stirring frequently, until the mushrooms start to brown and soften, 4 to 5 minutes. Add the garlic and stir for 1 minute.
Make the green bean casserole. Sprinkle the remaining 2 teaspoons flour into the pan and stir to break up any flour clumps. Cook for about 2 minutes and then stir in your chicken broth. As the broth steams and bubbles, stir vigorously and scrape the bottom of your pan to get up all of that browned goodness. After 1 more minute, stir in the half-and-half and bring to a boil. Reduce the heat to medium low and cook until thick and creamy, about 5 more minutes. Turn off the heat and gently stir the blanched green beans into the creamy mushroom sauce.
Smoke the green bean casserole. Top the green bean mixture with the crunchy onions and transfer, carefully, to your smoker. Place the skillet on the grill grates and close the lid. Smoke for 30 minutes.
Serve immediately. Remove the smoked green bean casserole from the smoker and serve while hot. Enjoy!