Preheat an oven to 400°F. Lightly grease a 9 x 13 baking dish with butter. Prepare an ice bath (a large bowl of cold water and ice). Set aside.

Add dried mushrooms to a small bowl with boiling water. Steep for at least 10 minutes. Once rehydrated, take the mushrooms from the water and roughly chop. Measure out 3/4 cup of steeping water, making sure not to include any grit that has settled at the bottom.

In a large skillet, warm butter over medium high heat. Once the butter starts foaming, add the diced mushrooms in an even layer, tossing to coat. Do not stir the mushrooms for 2 to 3 minutes. Once golden and caramelized, toss the mushrooms and cook for 2 minutes on the other side. Add the rehydrated mushrooms to the skillet along with a pinch of salt and pepper and cook for 1 minute, stirring occasionally.

Sprinkle the mushrooms with flour, stirring to coat. Pour in 3/4 cup of the mushroom steeping liquid and chicken stock, then stir to break up any clumps. Add the milk, reduce heat to medium and cook, stirring frequently, until the mixture has slightly thickened (about 4 to 5 minutes). Take the wild mushroom soup off the heat and season to taste with additional salt and pepper.

In a large bowl, toss together green beans, mushroom soup, bacon and most of the cheese. Season to taste with salt. Transfer mixture to the prepared baking dish. In a medium bowl, toss together breadcrumbs, butter, chives, salt and pepper. Top the casserole evenly with breadcrumbs, then sprinkle over remaining cheese.

Bake uncovered until the breadcrumbs are golden and sides are bubbling (20 to 25 minutes). Let stand for 5 minutes before serving.