Preheat oven to 450 degrees F. In a large bowl, add the flour, panko breadcrumbs, and salt.
Cut onions into thick slices. Add the onion to a large mixing bowl and mix to evenly coat the onions. Drizzle with olive oil.
Bake for 20 minutes, or until golden brown, stirring every 4-5 minutes. Remove from oven and let stand 2 to 3 minutes.
Melt butter in a large skillet over medium heat. Add onions and cook for 4 minutes. Add mushrooms and season with thyme, salt, and pepper.
Sprinkle the flour over the top and stir until combined. Stir in the chicken stock. Bring to a simmer.
Reduce the heat and add the half-and-half. Cook, stirring until the mixture has thickened. Remove from the heat and add the green beans. Mix well to combine.
Place skillet in the oven and top with the fried onions and bake for 15 minutes. Remove from oven and allow to cool.