Start by prepping the onion topping. Combine the buttermilk and Cattleman’s Grill Butcher House Brine in a quart container. Whisk well to combine. Add your thin liked onions to the container and submerge them in the buttermilk. Let soak until you’re ready to bread and fry the onions.
Preheat your Napoleon Prestige P500RSIB Gas Grill. Turn the outside burners to high. Turn the rear/rotisserie and inside/middle burners on low. Place a Napoleon Reversible Cast Iron Griddle in the center of the second shelf. This will act as a broiler to brown the top of the casserole.
Bring a medium sized pot of water to a boil on the infrared side burner. Blanch the green beans in the gently simmering water until tender, but not mushy, about 7 minutes. Remove with a slotted spoon and set aside.
Preheat a Lodge & Quart Dutch Oven inside grill while the green beans are being par-cooked.
Transfer the dutch oven to side burner. Add the diced bacon and cook until crispy. Remove the bacon from the dutch oven and place on paper towels.
Add the butter and melt. Add the mushrooms and onions. Season with Cattleman’s Grill Trail Dust All-Purpose Seasoning, Cook, stirring occasionally until most of the water has cooked out of the onions and mushrooms and they are starting to stick to the bottom, about 10 minutes.
Deglaze the cast iron with the white wine. Reduce until there is not visible liquid left in the bottom. Add the garlic and flour. Cook, stirring for one minute. Add the half and half and bring to simmer. Add parmesan and stir to melt. Turn off the heat. Add the green beans. Mix well.
Transfer the mixture to a Lodge Cast Iron Casserole Pan. Smooth out the top. Transfer to the grill, underneath the griddle. Cook for about 20 minutes.
Meanwhile, fry the onions on the side burner. Rinse out the dutch oven used to make the cream of mushroom. Wipe dry with towels and place on the side burner of the grill. Add vegetable oil for frying, about 1” deep. Preheat to about 400ºF.
Remove the onions from the buttermilk, a handful at a time. Dredge in the flour in a shallow pan. Transfer to a wire rack over a sheet pan. After all onions are breaded, begin frying.
Fry in 3-4 batches, cooking the onions just until golden brown, about one minute, then remove and transfer back to the wire rack.
Top the casserole with the fried onions and continue cooking another, 10 minutes, or until the casserole is bubbling and the top is browned. If the top begins to get too brown, turn off the rear infrared burner that is heating the griddle.