Caramelized Onion and Green Bean Casserole

Preheat the oven to 375°F (190°C).
Blanch the green beans in a pot of boiling water for 3-4 minutes, then transfer them to an ice bath to stop the cooking process.
Drain and set aside.
In a large skillet, heat olive oil over medium heat.
Add sliced onions and cook, stirring occasionally, until they caramelize and turn golden brown (about 15 minutes).
Add minced garlic, thyme, balsamic vinegar, and sliced mushrooms to the skillet.
Cook for an additional 5 minutes, until the mushrooms are tender.
Season with salt and black pepper.
Prepare the creamy sauce: In a saucepan, melt butter over medium heat.
Stir in the flour and cook for 1-2 minutes until it forms a paste.
Gradually whisk in the broth and milk.
Continue whisking until the mixture thickens.
Season with salt and white pepper to taste.
Combine the blanched green beans, caramelized onion-mushroom mixture, and creamy sauce in a large mixing bowl.
Mix until well coated.
Transfer the mixture to a baking dish and top with shredded Gruyère or Swiss cheese.
Bake in the preheated oven for 20 minutes or until the casserole is hot and bubbly, and the cheese is melted.
Remove from the oven, sprinkle crispy fried onions on top, and return to the oven for an additional 5-7 minutes, or until the onions are golden brown.
Garnish with chopped fresh parsley before serving.