Heat the oil in a frying pan. Add the leek and cook gently for 8 mins to soften. Steam or boil the broccoli for 2 mins. Drain and refresh in a bowl of cold water. Drain again. Preheat the oven to 190°C. Toss the broccoli in a shallow baking dish with the leeks, tomatoes, mozzarella, garlic and herbs. Season with salt and pepper. Scatter over the breadcrumbs. Bake for 20-25 mins, until the cheese has melted. Drizzle over a little olive oil to serve.