Sauté the mushrooms briskly for just a minute or so in 2 tablespoons of the olive oil, adding the crushed garlic and chopped parsley once the juices begin to run. Draw off the heat, cover with a lid and set aside.
Soften the onion in the rest of the oil over a gentle heat, covered, for 6-7 minutes. Then add the rice and stir until it is well coated with oil. Add a ladleful of the hot stock and simmer, stirring until it has been absorbed, then add another ladleful of stock. Continue adding the stock a little at a time and simmering until the rice absorbs the liquid before adding more.
When all of the stock has been added and the rice is fully cooked, stir in the cheese (optional) and tarragon. Season with mace, salt and pepper. Fold in the mushrooms with all their juices, and serve.