Bring a pot of water to a boil while you trim green beans, dice onions and bacon, and slice mushrooms.
Place a large wide skillet over medium high heat. Add bacon and stir occasionally until bacon comes to a crisp, about 10 to 15 minutes.
Meanwhile, fill a large bowl with water and ice and set near the stove. When water comes to a boil drop in green beans and blanch for 3 minutes. Remove green beans from water and place immediately into ice water to shock them and stop the cooking.
Add the onions in with the bacon and saute until softened, about 5 minutes, then add the mushrooms and saute another 5 minutes.
Remove green beans from water and gently blot dry to remove excess water. Add to the bacon, mushroom, onion mixture and cook another 5 to 10 minutes or until green beans are desired tenderness. Add salt and pepper, adjust to liking.
Serve immediately, and enjoy!
One Pan Instructions
To cook all in one pan, use a large, wide skillet with sides and begin by boiling the water in it. Once the green beans are blanched and in the ice water, dump out the water and return the pan to the heat. The remaining water will evaporate quickly and you can begin cooking the bacon.
Notes
*To substitute frozen green beans, do NOT thaw first. Skip the blanching and add them directly to the pan from frozen. Expect an additional 5-10 minutes of cook time.*To make this ahead, cook it completely, then transfer everything to a large casserole dish. Cool, cover, and refrigerate. Then uncover it and pop the dish into a 400°F oven for 10-20 minutes, stirring occasionally until everything is hot.