Preheat your oven to 375°F and set out a 9x13 casserole dish.

In a large saute pan, melt the butter and warm the olive oil over medium-high heat. Add the sliced white mushrooms and toss into the fats. Saute until lightly browned and soft, about 5-7 minutes.

Add the chopped yellow onion to the pan and toss into the mushrooms. Continue to saute until the onions are softened, about 3 minutes.

Add the minced garlic to the pan and saute until fragrant, about 30 seconds.

Reduce the heat to medium and add the haricot verts style green beans to the pan and toss with the aromatics and mushrooms. Let cook, carefully stirring every 2 minutes, until the green beans are fork tender, about 10 minutes. Once cooked, take off the heat.

To the saute pan, add the water and cream of mushroom soup. Toss to coat the green beans and vegetables with the soup and to integrate the water. Stir in 1 cup of the fried onion pieces until evenly distributed.

Spoon half of the green bean mixture into your 9x13 casserole dish. Sprinkle 3 oz of the shredded fontina, 3 oz of the gruyere cheese, and 1 tablespoon of the grated parmesan cheese.

Add the remaining half of the green bean mixture into the casserole dish. Top with the remaining fontina, gruyere, and parmesan cheese. Sprinkle the remaining ½ cup of fried onion pieces.

Bake the casserole in the oven for 15 minutes, until the cheese is melty and the soup mixture begins to lightly bubble.

Let cool on a cooling rack for 5 minutes and serve. Enjoy!