First, prepare the oven-fried onions by preheating the oven to 425 degrees F.
In a large bowl, toss the thinly sliced onions with the flour, panko, and coarse salt until evenly coated.
Spray a baking sheet with olive oil spray and spread the onions on the pan. Give them a spritz of olive oil and bake for 25-30 minutes, stirring every 8-10 minutes. Remove the baked onions and set aside. Reduce the oven heat to 400 degrees F.
In a large oven-proof skillet, melt the butter over medium-high heat.
Add the diced pancetta, mushrooms, salt, and pepper. Cook, stirring often, for 4-5 minutes, until the mushrooms begin to give up some of their liquid.
Add the garlic and nutmeg and cook for another 1-2 minutes.
Sprinkle the flour over the mushroom mixture, stir, and cook for 1 minute. Add the chicken stock and simmer for 1 minute more.
Lower the heat to medium-low and add the half and half and water chestnuts. Cook until the mixture thickens, stirring occasionally, about 6-8 minutes.
Remove the skillet from the heat, stir in the green beans and 1/4 of the onions. Top with the rest of the onions and bake in the oven for 15 minutes. Serve immediately.