Preheat the oven to 350 degrees.\nMelt butter in a medium sauté pan. Once melted, add in the mushrooms. Sauté until tender, about 3 minutes. \nAdd Southeast Asia sauce to the pan and whisk to combine. Add in the flour and stir until evenly distributed. Cook flour for about 1 minute, then whisk in the heavy cream. Continue cooking, allowing the sauce to thicken, about 2 minutes more. Add green beans to the pan and toss to evenly coat with sauce.\nPour green bean mixture into a casserole dish and bake until hot and green beans are cooked through, about 15 minutes. Remove from the oven, toss the green bean mixture to distribute the sauce and continue cooking until the top is bubbly and beginning to brown, about 5 minutes more. \nRemove from the oven and garnish with your favorite crispy onions. Serve hot!