Preheat the Oven:
Preheat your oven to 375°F (190°C) and grease a 9x13-inch baking dish.
Prepare the Green Beans:
If using fresh green beans, blanch them in boiling water for 3-4 minutes, then drain and set aside. If using frozen, cook according to package instructions and drain.
Make the Cheesy Sauce:
In a large skillet, melt butter over medium heat. Add the chopped onion and cook for 3-4 minutes, until softened.
Add the minced garlic and cook for another minute, until fragrant.
Stir in the heavy cream and chicken broth, bringing the mixture to a simmer. Allow it to cook for 3-5 minutes, thickening slightly.
Stir in the shredded cheddar and Parmesan cheeses, garlic powder, and onion powder. Season with salt and pepper to taste. Continue stirring until the cheese has melted and the sauce is smooth.
Combine Green Beans and Cheese Sauce:
Add the blanched green beans to the skillet with the cheese sauce, stirring gently to coat the beans evenly in the sauce.
Assemble the Casserole:
Pour the green bean and cheese mixture into the prepared baking dish. Smooth the top with a spatula.
Sprinkle the French fried onions over the top of the casserole. If desired, also add a little shredded mozzarella for extra cheesiness.
Bake:
Bake the casserole in the preheated oven for 20-25 minutes, until bubbly and golden on top.
Serve:
Let the casserole cool for 5 minutes before serving. It's perfect as a side dish for holiday dinners, potlucks, or a cozy family meal.