Clean and slice the mushrooms. Cut the onion into long strips. (I cut slices and then quarter the slices.)
Cook the green beans according to package directions.
In a large skillet, over medium heat, melt the butter. Add the onions. Sprinkle with salt and pepper. Add the mushrooms and garlic powder. Stir to coat. Cover the pan and cook for 5-10 minutes, until tender.
When the green beans are done, drain and add to the skillet. Toss with the mushrooms and onions. Serve warm.
Use fresh or frozen green beans for the best flavor.
You can add more garlic powder if you like.