Boil the Green Beans: Bring a large pot of salted water to a boil. Add the fresh green beans and cook for 6 minutes until bright green and crisp. Drain and run cold water over the beans.

Sauté the Aromatics: In a skillet, melt 2 tablespoons of butter. Add sliced onions and sauté for 5 minutes until browned. Stir in minced garlic and cook for 30 seconds. Add panko breadcrumbs and mix, then remove from skillet.

Create the Creamy Sauce: In the same skillet, add 1 cup of heavy cream and another tablespoon of butter over low heat. Stir until combined and heated gently.

Incorporate the Cheeses: Stir in the green beans, then add 1 cup of cheddar cheese and 1/2 cup of parmesan, stirring until melted.

Assemble and Bake: Preheat oven to 375°F. Pour the mixture into a greased casserole dish and top with reserved onion and breadcrumbs. Bake for 15-20 minutes until golden brown.