Preheat oven to 400Place green beans in a large bowl. Break off any unappealing ends.Drizzle with olive oil and sprinkle with a little salt and pepperMix until beans are coated.Place on a cookie sheet with parchment and roast for 20 minutes or until tender.Meanwhile, saute onions in some oil on low to medium heat until softened.Add mushrooms and cook until softened. Not too long though, you dont want them to be mush.Set mushrooms aside.Place garlic and butter in a saucepan on low heat and heat until butter has melted.Add flour and stir into butter with a spoon. It will become a thick paste. Stir for about 30 seconds to cook out flour taste.Slowly add milk , a couple tbs at a time, stirring.Once milk is combined turn heat to low simmer.Stir consistently until sauce thickens, maybe 5 minutes, maybe less.Once sauce has thickened, turn off heat and add mushroom mixture.Pour mushroom sauce over roasted green beans!