Boil a pot of salted water. Chop green beans into smaller bite sized pieces - about 1.5 inches long each

Place green beans in the boiled water for 2 minutes. Remove from the pot and immediately drain and rinse with cold water to stop them from cooking.

Preheat the oven to 350F and grease a 9x13 casserole dish.

Heat a skillet over medium heat and add olive oil. When the oil is warm, add the chopped mushrooms and minced garlic. Saute until softened - about 5 minutes. Mix in salt.

Reduce heat to med-low and add butter to the skillet. Once the butter has melted, add the flour and stir it in. A thick paste will be created.

Slowly mix in the milk and dijon mustard, mixing constantly to avoid any clumps. Turn off heat once the milk is fully incorporated.

Place green beans in a greased casserole dish and stir in the mushroom mixture. Stir in ½ cup of crispy fried onions.

Place in the oven to bake uncovered at 350F for 30 minutes. Remove from the oven and top with 1 ½ cups of the crispy fried onions. Return to the oven and bake for another 5-10 minutes. Serve warm.