In the meantime, heat a large saucepan over medium heat. Once hot, add half the olive oil (1 Tbsp as original recipe is written) and mushrooms. Season with a pinch each salt and pepper and sauté until tender and slightly browned - 3-4 minutes - stirring frequently. Remove from pan and set aside in a small dish.
Heat the same large saucepan over medium heat once more. Once hot, add remaining olive oil (1 Tbsp as original recipe is written // adjust if altering batch size) and leeks. Sauté for 1-2 minutes, or until softened and very slightly browned.
Continue to add vegetable stock, stirring to incorporate, until the rice is 'al dente' - cooked through but still has a slight bite. This whole process should only take 15-20 minutes (time based on original recipe // adjust if altering batch size).