Blanch the green beans for 2 minutes, then either chop or process your mushrooms until you have a mince (Note: see bottom of instructions for steps on frying unions. This is an important step in to be completed before starting the dish, or while in the oven.)
Preheat a pan over medium heat and add a few tablespoons of oil, once shimmering add your mushrooms.
Cook for close to 15 minutes or until golden then add butter onions and aromatics and sauté for 5 minutes until onions are translucent.
Sprinkle in flour and cook for an additional 3 minutes add a splash of milk to help scrape up fond then add the rest of the milk and heavy cream.
Simmer for 20 minutes, then slowly add the cheese and mix in so as not to get a clumpy sauce.
Once incorporated salt and pepper to taste and pour over green beans and mix well. (Note: if prepping in advance stop here. Cover and refrigerate, resume following day.)
Preheat oven 350F/176C cover casserole with an even layer of breadcrumbs bake for 15 minutes or until golden(Note: mix crumbs with melted butter prior, it assists with browning and adds to crunch)
Top with fried onions and serve
** Shallot instructions **
Slice shallots ¼” thick place in a pot and cover with a neutral oil place over medium heat and if using a thermometer try to keep around 350F/176C. Stir often as your shallots will burn, once they start to turn golden remove from heat and strain. They will continue to darken as they cool.
