If using canned string beans proceed to step 4. Wash and cut string beans up to desired size
Boil string beans to soften. About 5-10 minutes. Don't overcook because they will cook in the oven as well.
After finish boiling drain them then make cream sauce for casserole.
Sliced onion and mushrooms. On medium heat sweat the mushrooms to remove excess water.
After they are done add in olive oil and toss in sliced onions then season with salt and pepper.
Cook mushrooms and onions down then add in butter and allow to melt then incorporate.
Pour in white wine vinegar and allow the vinegar to completley reduce making sure you don't burn anything watch the heat. If onions/mushrooms look dry add a little oil but they shouldn't.
After vinegar has reduced add broth to the pan. Bring to boil then lower to simmer and reduce by half.
Pour in heavy cream and reduce to simmer as well. Allow to thicken. If you want a thicker sauce since there is no roux add in 1/8 tsp xanthan gum or 1/4 cup of cheese of choice.
Preheat oven to 300ºF then layer string beans in casserole dish.
Mix in cream sauce and string beans then bake for 20 minutes. Allow to cool then serve.