The book encourages you to cook with rapeseed oil instead of olive oil and to use olive oil just for the finish - totally buying into that. Besides, dairies are used sparingly, mostly for flavoring what I welcome.
As timeless and classic this dish is, it was featured in the River Cottage series for a wedding menu so there was an unconventional approach behind it :)
The ragout is insanely delicious and as any ragout it is as good / even better the next day. So processing double of the amount of mushrooms as suggested (at least 1 kg but you won't regret 2 kg - they shrink after cooking) in order to yield to leftovers (for a sandwich, a salad or a quick polenta*) is a must!
You can find the recipe here.
Additional remarks: it only serves 4 if served within a five-courses-menu :)
For a single-course meal it serves 2, at most.
If you prefer a creamy polenta use 50 g less polenta than suggested in the recipe.
*How to cook instant polenta: bring water with a generous amount of cream to boil (400 ml water, 100 ml cream for 90 gr polenta). Add salt and pepper. Add the polenta, turn down the heat, stir for a couple of minutes until all water is absorbed. Add grated parmigiano. That's it.