Preheat the oven to 350 F (180 C). Grease a 7 inch (18 cm) round cake pan.
PREPARE THE CAULIFLOWER RICE
Cut the head of cauliflower into quarters, then trim out the inner core from each quarter. Break apart the cauliflower into large florets with your hands. If the core is tender, you can break it into pieces and add it with the florets.
Transfer the cauliflower to a food processor. Don’t fill the food processor more than 3/4 full; if necessary, process it in two batches.
Process the cauliflower in 1-second pulses until it has completely broken down into rice-sized granules.
In a large pan, heat 2 tbsp of olive oil and fry the onions and garlic gently until well softened.
Stir in the cauliflower rice, then pour in 1 cup of vegetable stock. Stir and cook for about 5 minutes, until the cauliflower rice is tender but still crunchy. Set aside.
In a small pan, heat 1 tbsp of olive oil, sauté the mushrooms and the chopped thyme leaves until golden and tender.
Scrape the cooled cauliflower rice into a large bowl with most of the mushrooms, leaving a few to top the cake later.
Add all the parmesan, ricotta and beaten eggs. Season with plenty of salt and black pepper and mix well.
Tip the cauliflower rice mix into the cake pan and press out firmly to smooth the top.
Arrange the remaining mushrooms on top and scatter over the fresh thyme sprigs, drizzle with a little olive oil.
Bake for 30-35 minutes until golden brown. Cool for 20 minutes and enjoy!