Preheat oven to 375°F. Steam or boil green beans until just tender, 4-6 minutes, then drain.
Heat oil in a skillet over medium. Cook onion 4 minutes until soft, then add garlic and cook 30 seconds.
Stir in tomatoes, broth, oregano, and red pepper flakes. Simmer 5 minutes to thicken slightly. Season with salt and pepper.
Toss beans with tomato mixture in a baking dish. Sprinkle mozzarella and Parmesan on top.
Mix panko with melted butter and sprinkle over cheese.
Bake 15-20 minutes until cheese melts and crumbs are golden.