Preheat oven to 350 degrees.
If using fresh green beans, boil water in a sauce pan over medium-high heat and cook green beans for about 5 minutes. Drain and rinse under cold water and set aside.
If using canned green beans, drain and set aside.
Fry diced bacon in a large skillet until crispy. Transfer to a plate lined wiht paper towels. Leave about 1 tbsp of bacon grease in the skillet, removing excess.
ADd mushrooms to the leftover bacon grease and cook for several minutes or until they begin to soften. Add minced garlic to the pan and continue to cook with the miushrooms for 2-3 minutes.
In a large bowl, combine all ingredients, reserving 1/2 cup of the fried onions and a handful of bacon pieces. Pour into a 2-quart casserole dish and smooth the top.
Bake at 350 degrees for 30 minutes or until mixture is hot and bubbling.
Add remaining fried onions and bacon on top of casserole and cook for an additional 5-10 minutes.