Place the porcini mushrooms in a small bowl and just cover with a little boiling water. Set aside.
In a large non-stick pan, melt the butter over a low heat. Add the garlic, onion and celery and cook with the lid on for a few minutes just to sweat and soften (not too brown).
Add the chicken and continue to cook for a few more minutes until the chicken just starts to turn opaque.
Remove the porcini mushrooms from the water, reserving the soaking water.
Chop the porcini mushrooms then add to the pan with the rest of the mushrooms, zucchini, and thyme leaves and cook for a few minutes until the mushroom starts to soften.
Add the rice and a splash of white wine, the reserved porcini liquid, followed by the stock and simmer for 5 minutes, stirring often until the stock is almost all absorbed.
Turn off the heat and stir in the spinach and cheese. Once the spinach has wilted and the cheese melted through the dish, top with parsley and serve immediately.