Heat oven to 200C/180C fan/gas 6 and place a baking sheet inside to get hot. Heat the oil in a large non-stick frying pan and cook the mushrooms and garlic, stirring frequently, until softened. Add the spinach, a handful at a time, and cook until just wilted but not soggy. Add the onion and stir it through, then turn off the heat.
For the base, tip the flour and baking powder into a bowl, and stir in the yogurt mixture with the blade of a cutlery knife to make a ball of soft dough. Dust a sheet of baking parchment with flour, put the dough on top and press with floured hands to make a flat 20cm round. Top with the spinach mixture, scatter over the cheese and walnuts, then bake on the hot baking sheet for 12 mins or until the dough is cooked through and golden.