Preheat the oven to Gas Mark 4, 180°C (150°F).
Melt a knob of butter with a dash of olive oil in a wide, shallow ovenproof saucepan over a gentle heat. Add the onion and cook until softened, but not browned. This will take about 5 minutes. Add in the rice and stir for 1-2 minutes until it starts to look translucent. Add the wine and stir until it evaporates.
Pour in enough of the heated stock to just cover the rice and simmer gently, stirring now and again. Season with a little salt and pepper. As the stock evaporates add more and stir intermittently until all the stock is used. This will take about 15 minutes. Taste regularly and add a little more salt if necessary. Stir in the frozen vegetables and basil leaves. Then season the fish and place it in a single layer on top of the risotto. Cover and place in the preheated oven for 10-12 minutes until the fish flakes easily with a fork. Remove from the oven and serve immediately with a side salad.