Preheat the oven to 350°F. Bring a large pot of salted water to a boil and set aside a bowl of ice water.

Trim the ends of the green beans, then drop them into the boiling water and cook until bright green and crisp-tender, 2 minutes. Quickly drain and add to the bowl of ice water to stop the cooking process. Drain and set aside.

Heat the olive oil in a large pan over medium heat. Add the onion and sauté until softened, 3 to 4 minutes. Add the garlic and continue cooking until the mixture is pale gold in color, 2 minutes longer.

Add the tomatoes and oregano, bring to a simmer and cook until most of the liquid has evaporated, 4 to 5 minutes more. Combine with the beans, season to taste with salt and pepper and transfer to a casserole dish coated with nonstick spray.

Bake for 25 to 35 minutes, or until the beans are tender when pierced with a fork. Remove from the oven and increase the temperature to 400°F. Combine the melted butter, panko crumbs and parmesan cheese in a small bowl and sprinkle the mixture evenly over the beans. Return to the oven and bake until the crumbs are lightly toasted, 4 to 5 minutes.