Sauté the onions and stir in the garlic and sauté until fragrant. Add the mushrooms and cook until well-browned and softened. About 5 minutes.
Remove the mushrooms from the skillet. Add the buttert. Whisk in the flour to form a roux. Add the white wine, and milk to form a bechamel.
Return the mushrooms and onions to the skillet along with the mustard, horseradish, and salt. Stir to combine.
Bring a large pot of water to a boil. Salt it generously and add the green beans. Cook for no more than 3 minutes before transferring to an ice bath to stop the cooking process.
Drain the green beans and place them in the mushroom bechamel. Toss to coat. Transfer the beans to an 11×7" casserole dish and bake for 15-20 minutes.
In a large bowl whisk to combine the chickpea flour, baking powder, and salt. Add the chilis, cilantro, and onions. Toss to coat. Slowly stir in the warm water until a batter forms.
Using tongs, grab onion pieces and add them to the oil. Sprinkle the onions with salt and place them on top of the green bean casserole fresh from the oven. Serve immediately.