Place the stock in a saucepan and bring to a boil. Reduce the heat slightly and cook until reduced by half.
Heat the oil in a frying pan over medium heat and fry onion and garlic until soft. Add mushrooms and cook 1-2 minutes until softened. Add tomatoes, tomato paste, and tomato sauce and reduced stock. Bring to a boil over high heat, reduce heat and simmer 10-15 minutes or until the mixture thickens. Stir in basil leaves.