1. Heat the oil and butter in a large frying pan,then add the leek,mushroom and ½ tsp salt. Saute on medium heat for 5 minutes,until the vegetables have softened,then add the asparagus,garlic and thyme. Cook for another 5-7 minutes until the mushrooms give up their moisture and begin to brown. Remove from heat and set aside to cool a little.
2. Bring a full kettle of water to a boil. Place the baby spinach leaves in a large heatproof bowl and pour boiling water over to cover. Leave for 1 minute,then drain in a sieve and cool under cold running water. Squeeze the spinach firmly to remove excess water,then chop finely and add to the frying pan with the cooling leeks,mushrooms and asparagus.
3. Preheat oven to 175C fan-forced (195C conventional). Brush the inside of a loaf tin (20cm x 11cm x 6cm,or 1.4L) all over with the 1 tbsp extra softened butter. Combine the walnut and the 20g extra parmesan in a small bowl,then tip into the tin. Shake and tap to coat the bottom and sides,then tip the excess back into the small bowl and reserve for the topping. Place the tin in the fridge for now.
4. Sift the flour,baking powder,baking soda and remaining ½ tsp salt in a medium bowl.
5. Place the eggs and yoghurt (or sour cream) in a large mixing bowl and whisk well until thoroughly combined. Add the 100ml olive oil and continue to mix until fully incorporated,then add the vegetable mixture,the 60g parmesan,za'atar and sifted flour mixture. Stir and fold with a silicone spatula until just combined – there should be no floury streaks visible in the batter,but be careful not to over-mix as it will produce a rubbery texture.
6. Remove the loaf tin from the fridge and scrape the batter into the tin. Scatter the reserved walnut and parmesan crumbs on top and press lightly so they stick to the batter. Place in the preheated oven and bake for about 55 minutes or until a skewer inserted into the middle of the loaf comes out clean.
7. Remove from the oven and allow to cool in the tin for about 15 minutes before loosening the edges with a knife. Place a large plate on top and carefully invert the loaf to dislodge it from the tin,then flip again onto another plate so it is the right side up. Serve at room temperature and spread with softened salted butter.
*Everything you need to know about zaatar