Start with the polenta. Put a large saucepan on a medium heat and add the milk and salt. Gradually pour in the polenta, whisking all the time. Turn the heat down to low and cook for 15-20 minutes while you cook the stew, whisking regularly.
In another large pan, add the olive oil. Peel and finely chop the onion - add to the pan and cook on a medium heat for two minutes.
Cut the mushrooms in half and add to the pan: cook for another 2-3 minutes.
Slice the sausages into rounds and add to the pan, along with the lentils (drained), passata, rosemary and thyme. Simmer for ten minutes.
Return to the polenta. Add the butter and cheese and whisk until it is melted and well combined. Taste and add more salt and some pepper if needed.
To serve, spoon the polenta into bowls and top with a ladleful of the sausage stew.