1. Bring vegetable stock to a low simmer in a large saucepan.
2. Heat the oil in a large heavy saucepan over medium heat.
3. Add chopped onion, garlic, and thyme and sauté for 2 minutes, or until onions are tender but not browned.
4. Add the rice and sauté for 30 seconds to coat with the oil.
5. Stir in the wine and cook for 3 minutes.
6. Add stock and cook for about 20 minutes, or until the liquid is absorbed and the rice is dente (the centre of each grain should be slightly firm).
7. Add sliced mushrooms and cook for 2 minutes.
8. Add thyme leaves, discard stems.
9. Remove risotto from heat, add butter and grated parmesan.
10. Stir until butter has melted.
11. Finally stir in parsley, mint and lemon juice, and season generously with salt and pepper.
Serve the risotto in 4 warmed serving bowls. Garnish with the shaved parmesan and serve.