How to Make It
Preheat the oven to 425° and position a rack in the center. In a large, deep skillet, melt the butter in 2 tablespoons of the oil. Add the onion and garlic and cook over moderate heat, stirring, until softened, 5 minutes. Add the bell pepper, corn, green beans, fava beans and Aleppo, season with salt and black pepper and cook, stirring, until the beans are crisp-tender, 3 minutes. Add the stock and simmer until evaporated, about 3 minutes.
Transfer the vegetables to a large baking dish and let cool slightly. Stir in the basil, chives, cream and eggs.
In a small bowl, toss the panko with the shredded cheese and the remaining 2 tablespoons of oil. Season the topping with salt and pepper; sprinkle over the succotash and bake for 10 minutes, until heated through.
Turn on the broiler and broil in the center of the oven for about 3 minutes, just until the top is golden. Let the gratin stand for 10 minutes, then serve.