Put courgettes in roasting dish in a single layer. Toss with 2 tbsp olive oil and season. Roast for 10 minutes then add remaining olive oil and spring onions. Return to the oven and roast for another 10-15 minutes. The courgettes should be tender and the spring onions slightly scorched.
Reduce oven to 170 C fan.
In a large bowl mix all the other ingredients and season well. Spoon the vegetables into a gratin dish with a capacity of a litre. Pour the batter over the vegetables and bake in oven for 15-20 minutes or until just set, golden and slightly puffy. Serve hot or warm, scattered with dill.