• Preheat oven to 375° F.
• Rinse and trim green beans. Cook according to your texture preference. For a more crisp green bean, boil for 5 minutes. For a softer green bean, boil for 10 minutes or cook in the Instant Pot for '0' minutes. Once cooked, place green beans in a 9x9 baking dish.

Sauce

• Melt butter in a saucepan over medium-low heat. Whisk in flour and continue whisking until the mixture is smooth and bubbly, making a roux.
• Remove the saucepan from heat and slowly add in chicken broth and milk, whisking continuously. Whisk until combined, then return to the heat. Add in 2 tbsp chopped onion and all the mushrooms.
• Bring to a gentle boil and simmer for 5-10 minutes, until thickened. Mixture will continue to thicken as it cools. Salt and pepper to taste. Stir in shredded cheese until melted.

Crispy Onion Topping

• Saute onions in 1 tbsp of butter until they are soft. Set onions aside.
• In the same pan, add remaining butter and saute the panko until browned. Stir the onions and panko together.
• Time to assemble! Stir the creamy sauce and green beans together and place in your baking dish. Sprinkle the crispy onion topping on top.
• Place in the oven, uncovered, and bake for 25-30 minutes, until the top is golden brown. Serve warm.