Preheat your oven to 350°F (175°C).

In a large skillet, melt 2 tablespoons of butter over medium heat. Add 1 diced onion and 8 ounces of sliced button mushrooms, sautéing for 5 to 7 minutes until tender and golden. Stir in 2 minced garlic cloves and cook for an additional 1 to 2 minutes until fragrant. Remove from heat.

Bring a pot of low-sodium chicken broth to a boil, then add your trimmed fresh green beans. Blanch them for about 8 minutes until crisp-tender. Drain in a colander and transfer to a bowl of ice water to halt cooking.

In a large mixing bowl, combine the sautéed vegetable mixture with 1 can of low-sodium cream of mushroom soup, drained green beans, and 1 cup of grated Parmesan cheese. Season with salt and black pepper to taste, and stir until well combined.

Lightly grease a 2-quart baking dish. Pour the green bean mixture into the dish and spread evenly.

Sprinkle crispy fried onion strings over the casserole.

Bake in the preheated oven for about 20 minutes until bubbly and golden brown. Let cool slightly before serving.