Directions 

Prepare the filling:

1. Preheat the oven to 375°F. 
2. Heat a large stockpot of water over high heat, until boiling. 
3. Wash, trim, and cut the green beans.
4. Add 2 Tablespoons salt to the boiling water. 
5. Prepare an ice bath by adding ice and water to a large bowl.
6. Add the green beans to the boiling water and boil for 3 minutes, or until the green color is vibrant. 
7. Use a slotted spoon to transfer the green beans to an ice bath.
8. Remove green beans from the ice bath after 1-2 minutes, or until the green beans have fully cooled. Transfer to a dish towel to dry and remove excess water. 
9. Prepare the onion. Begin by heating ½ Tablespoon olive oil in a large sauté pan over medium heat. 
10. Add the sliced onion and sauté for 2-3 minutes, or until the color begins to dull.
11. Remove onions from the pan and set aside in a bowl. 

Prepare the crispy topping:

1. Add ½ Tablespoon olive oil to the sauté pan, and add the panko breadcrumbs.
2. Sauté for 2-3 minutes, stirring often, until breadcrumbs begin to brown.
3. Transfer the bread crumbs to a separate bowl, and add Parmesan cheese, salt, and pepper. Stir to combine.
4. Rinse and dry the sauté pan to use to prepare the mushroom sauce. 

Prepare the mushroom sauce:

1. Begin by heating 2 Tablespoons olive oil in a large sauté pan over medium heat. Add sage to the olive oil while heating to infuse flavor, and remove before adding mushrooms (optional).  
2. Add the mushrooms and ½ teaspoon salt, and sauté for 5 minutes, or until mushrooms are browned and reduced by at least half. Stir occasionally.
3. Add the garlic and sauté for 1-2 minutes. Stir occasionally.
4. Stir in the flour and sauté for about 1 minute, or until combined.
5. Add in the vegetable stock and stir until the flour is combined.
6. Allow milk to get to room temperature. Slowly stir in the milk until combined.
7. Add grated Parmesan cheese and stir until combined. 
8. Cook sauce until it begins to thicken, about 5 minutes, stirring occasionally. Add sage to sauce while it’s thickening to infuse flavor, and remove before adding beans (optional). 

1. Turn off the heat. Taste, and add salt and pepper to taste.
2. Add the green beans to the sauce and stir until combined.

Assemble the casserole:

1. In a 9×13” baking dish, evenly layer the green bean sauce mixture, then the sliced onion, then the crispy topping. 
2. Bake for about 25-30 minutes, or until the topping is browned to your liking. Top with aluminum foil before baking to keep topping from burning if needed. 
3. Allow at least 10 minutes to cool and thicken before serving.
4. Garnish with additional black pepper, parsley, and fresh lemon juice (optional).