Bring a large pan of salted water to the boil. Add the rice and simmer for 15-20 minutes untilalmost fully cooked then drain.
Meanwhile, heat the butter in a large frying pan and cook the onion for 8 minuts. Increase the heat to high , add the courgettes, and cook for 10 minutes. Stir in the garlic and add some seasoning, then combine with the rice and set aside to cool.
Preheat the oven to 200 C, 180 Fan, Gas 6. In a bowl mix together the mascarpone, milk, mustard, 75g of the Gruyere and some seasoning. Add to the rice and mix well.
Transfer to a lightly buttered baking dish and top with the remaining cheese. Bake for 30 minutes until golden and bubbling.
Meanwhile, mix together the ingredients for the salad. SErve alongside the gratin.