In a saucepan, melt the butter over low heat. Add the onion and cook until soft, about 10 minutes. Add capsicum and mushrooms and cook another 5 minutes, stirring frequently.
Meanwhile, in a separate saucepan, heat the stock, salt and water to a simmer.
Add the rice to the onion and pumpkin and stir until rice is coated. Add wine and simmer 2 minutes. Stir in chicken.
Add 1/2 cup stock and stir until absorbed. Continue adding stock mixture 1/2 a cup at a time, stirring until absorbed. The rice should take about 25 minutes to cook completely.