Place oil in a pot set over medium heat. Add the leek, red pepper, mushrooms and garlic and cook until tender, about 6 to 7 minutes. Mix in the rice and tarragon. Add the stock, increase the heat, and bring to a boil. Once it boils, cover, and turn the heat to its lowest setting. Cook 15 minutes, or until the rice is tender. Mix in the parsley and serve.