Place the stock into a large pan over a medium heat and bring to the boil, then turn the heat down as low as it will go, to keep the stock hot but without letting it reduce too much.
Heat 30g/1oz of the butter in a wide pan over a low to moderate heat. When the butter is foaming, add the onion.
Using a grater, coarsely grate one-third of the courgettes. Add the grated courgettes to the pan with the onion and fry gently, without browning, until both are tender.
Melt 15g/½oz of the butter with the olive oil in a frying pan over a high heat. Chop the remaining courgettes into 1cm/¼in cubes. Add the chopped courgettes to the pan with the butter and olive oil and saute over a medium to high heat, until they're tender and beginning to turn brown. Remove from the pan and keep warm.
Add the rice to the pan with the onion and grated courgettes and stir for 30-60 seconds to mix well. The rice should become translucent.
Pour in the white wine. Simmer until the wine is absorbed.
Chop the herbs finely. You should aim for about two tablespoons each of chopped fresh parsley, basil, chives and mint and half a tablespoon of chopped fresh tarragon.
Season the risotto with salt and pepper. When the wine is all absorbed, add a ladleful of the hot stock and keep stirring, until that has been absorbed. Keep adding the stock in the same way, stirring as frequently as you can, until the rice is cooked al dente (that is, tender but still with a slight resistance to the bite). This will take 20 minutes or more. If you run out of stock before that, just add boiling hot water. At this point, the risotto should still be fairly wet, and moist, but not swimming about in a lake of liquid.
When the rice is cooked, stir the sauteed courgettes and most of the chopped herbs (keeping aside a sprinkling for serving the final dish) into the risotto and cook for a further 1-2 minutes to heat through.
Finally, stir in the last of the butter and the parmesan. Season, to taste, with salt and freshly ground black pepper and serve, sprinkled with the reserved chopped herbs.