Preheat the oven to 350 degrees.
Cook the 'steam in bag' green beans in the microwave according to package directions. Set aside.
Melt 2 to 3 tablespoons of butter in a large skillet over medium heat. Add the onion; cook and stir until onion is starting to become tender, about 5 minutes. Stir in the chopped mushrooms. Sauté until the veggies are brown & caramelized. Season with salt & pepper. Remove the veggies from the pan & set aside.
Melt 3 tablespoons of butter in the pan, scraping any browned bits up with a wooden spatula. Whisk in the flour until blended & cook for about 30 seconds to 'activate' the flour. Whisk in milk, stirring constantly, so that no lumps form and then stir in the onion dip & garlic powder. Warm over medium heat until the sauce begins to bubble. Turn off the heat & dump in the green beans and sautéed onions & mushrooms, stirring to coat.
Stir in the cheddar cheese and half of the French fried onions. Taste with a little spoon to make sure you don't need any more salt or pepper - if you do, add it to taste.
Pour into a 2 quart or 9x13 casserole dish. Top with the remaining French fried onions.
Bake at 350 for 30 minutes.