Heat the oven to 180C and oil a cast iron pan or oven-safe dish.
In a pan fry the onions and garlic until translucent. Add in the mushrooms, thyme, salt and pepper and keep frying. Turn the heat down and stir in the flour and then the milk and allow it to thicken.
Set aside and allow it to cool.
Layout the puff pastry and spread some mushroom mix in the centre, then place the cauliflower upside down on the mushroom mix and pack the rest of the mushroom mix on the cauliflower. If you struggle to do that you can spread it on the pastry, just make sure you leave a gap so that the pastry can stick to itself.
Wrap the pastry around to completely cover it. Then turn it upside down and place it in the oiled cast iron pan.
Decorate however you choose, brush with soy milk and place in the oven. It will be ready when the pastry is browned and flaky.
Take it out and allow it to sit for a while to steam before serving.