Preheat oven to 350 degrees
Thaw the bag of frozen french cut green beans, place in an 8x8 baking dish & set aside
In a deep pan, heat 4 tablespoons of butter & oil over medium-high heat
Add in mushrooms & shallot, saute until they start to turn golden
Add remaining butter & sprinkle over flour & stir to coat, reduce heat to medium
Stir to create a roux, 1 minute
Add in the stock & stir until flour is dissolved, you should have a pretty thick sauce base
Add in half & half plus milk, whisk to combine. Keep stirring to avoid burning or sticking to the bottom of the pan
Add in salt & pepper to taste along with a dash of nutmeg
Remove from heat & pour over thawed green beans
Gently mix together
Bake in a preheated oven (350 degrees) covered, until bubbly. Roughly 30 minutes (more if using pre-prepared cream of mushroom soup)
Remove from the oven & cover with the crispy onions
Bake uncovered for 5 minutes, keeping a close eye on the onions, they will turn golden very quickly, avoid burning them!
Remove from oven & let sit for 5-10 minutes before serving
Enjoy!