Heat a splash of oil in a large saucepan over medium-high heat. Add the mushrooms and cook until almost browned, stirring occasionally, 5 mins. Add the shallot, tomato purée and remaining rosemary and cook for another 2-3 mins, stirring often. Stir in the basmati rice and then pour in the stock. Bring to a simmer, lower the heat to medium, cover and leave to cook for 10 mins.