Heat the oil in a very large frying pan over a medium heat and cook the onion for 5-6 minutes until soft. Add the garlic and continue to cook for a further 1-2 minutes.
When the dried mushrooms are soft, remove them from the hot water with a slotted spoon, reserving the liquid. Add them to the pan along with all the other mushrooms and the fresh thyme. Cook for 4-5 minutes until the mushrooms have started to cook down. Sprinkle over the flour and continue to cook for a further 2-3 minutes. Add the reserved mushroom liquid and bring to a simmer. Cook for 10-12 minutes, stirring frequently, until the cooking liquid has reduced to a thick sauce.
Add the spinach to the pan. Leave to wilt for a couple of minutes, then stir through. Season with black pepper.
Meanwhile, cook the sweet potato tagliatelle according to the instructions on the packet. Serve the tagliatelle with mushroom ragu topped with the Italian hard cheese.