Cover the porcini with boiling water (about 200ml) in a bowl; set aside to soak for 20 minutes.
Meanwhile, heat the oil in a wide pan then cook the shallots with a pinch of salt over a medium heat for 6-8 minutes until tender and light golden.
Turn the heat up to high then add the chestnut mushrooms. Cook for 3 minutes without stirring.
Add the sliced garlic and stir in 1 tbsp water. Cover and take off the heat; set aside to steam in the juices for 10 minutes.
Meanwhile, bring 1 litre of water to a simmer; add a pinch of salt.
Pour in the polenta in a steady stream then cook on a low heat for 8-10 minutes, whisking all the time.
Add the butter, cheese and rosemary; stir vigorously until smooth and combined. Season.
Stir the porcini and soaking liquor into the garlic mushrooms. Add the lentils and warm through over a low heat; season.
Serve with the polenta and a scattering of parsley.