Bring a large pot of salted water to a boil. Carefully add the fresh green beans and cook for about 6 minutes, until they are bright green and tender-crisp. Drain and rinse under cold water to stop cooking. Set aside.

In a large skillet, melt 3 tablespoons of butter over medium heat. Add sliced onion and sauté until golden brown, about 5 minutes. Add minced garlic and cook for 1 more minute.

Stir in the panko breadcrumbs and allow them to toast slightly. Remove from heat to cool.

In the same skillet, return it to low heat and pour in the heavy cream along with the remaining butter. Heat gently, stirring until smooth.

Stir in the blanched green beans into the warm cheese sauce, then gradually add the cheddar and parmesan cheeses, mixing until fully melted and creamy. Adjust seasoning with sea salt.

Preheat your oven to 350°F (175°C). Pour the mixture into a greased baking dish and sprinkle the toasted onion and panko mixture on top.

Bake for about 15 minutes, or until the top is golden brown and bubbling. Let cool slightly before serving.