Step 1

Preheat a large sauté pan over medium high heat, then melt 1 tablespoon butter. Add onions, and cook for 10 minutes or until onions have wilted considerably. Add salt and sugar and cook until caramelized. Once onions are a light caramel brown color, about 50 minutes to 1 hour of cooking, add one tablespoon of mirin. Quickly deglaze the pan by scraping up any browned bits from the bottom of the pan and mix them into the caramelized onions. Once liquid has completely evaporated, transfer onions to a clean bowl and set aside. Return sauté pan to the stove for later use.

Step 2

Meanwhile, preheat oven to 350°F. Fill a large stock pot with water, and bring to a boil over high heat. Salt until water tastes of tear drops, about two tablespoons.

Step 3

Preheat the large sauté pan over medium high heat. Melt butter, then add mushrooms. Do not move mushrooms and allow them to brown on one side, about 3 to 5 minutes. When mushrooms begin to brown, quickly flip them, and allow to brown on the other side, about 3 to 5 minutes. Once mushrooms have wilted and turned a caramel brown color, add 1 tablespoon of mirin and deglaze the pan again. Add cream of mushroom soup, vegetable broth, and soy sauce. Turn down heat to medium low.

Step 4

In the large stock pot, add green beans to boiling, salted water. Cook until tender, then drain the beans in a colander, and stir into mushroom soup mixture. Mixture should be thick, but if it appears too runny, turn heat up to medium high and cook until mixture reduces down. Salt to taste. Then, pour the contents of the sauté pan in a buttered casserole dish. Mix panko or breadcrumbs with caramelized onions, and sprinkle over casserole. Bake until mixture bubbles and is golden brown on top, about 25 minutes.