To make the za’atar oil: Add the za’atar and extra-virgin olive oil to a small saucepan. Place over low heat and heat just until the oil is hot—this will take about a minute. Immediately remove from the heat. Stay close to the stove while you do this step to make sure you don’t burn the za’atar! Cover the saucepan and set aside to infuse while you make the soup. (You can also make the za’atar oil in advance, let cool to room temperature, store in an airtight container and refrigerate until ready to use—or up to a week.)

To make the soup: In a large saucepan, heat the olive oil over low heat until it shimmers. Add the shallots and garlic and sweat until translucent, about 5 minutes. Keep the heat low to make sure the shallots and garlic don’t brown.

While the shallots and garlic are cooking, roughly chop the cauliflower head, removing the tougher bottom of the stem but keeping the rest. Add to the saucepan, sprinkle with a generous pinch of sea salt and a grind of black pepper, then stir and cook for a minute. Add the chicken stock and the sprigs of thyme (no need to remove the leaves from the stems, they will fall away during cooking). Bring to a simmer, cover, and cook over low heat for 15 minutes, or until the cauliflower is very tender.

While the soup simmers, chop the spinach: gather a handful of leaves, roll it up into a tight bunch, then chop finely. Repeat for the rest of the spinach. This step is important because it will allow the spinach to cook very quickly and keep its bright color.

When the cauliflower is tender, turn the heat up to medium, then add the spinach to the saucepan. Stir to incorporate the spinach to the soup and leave it to simmer for 2 minutes—don’t leave it for any longer or the color of the spinach will turn dark and brownish.

Remove the saucepan from the heat. Carefully transfer half of the soup to the container of a blender, then add the tahini. Blend until the soup is silky smooth. Transfer to a large measuring cup, a large bowl, or a second saucepan. Blend the second half of the soup, then combine both batches of blended soups. Stir well to fully incorporate the flavors, then taste and adjust seasoning if needed. Keep the soup hot until ready to serve.

To serve: Divide between serving bowls. Top each serving with a dollop of Greek yogurt, a generous drizzle of za’atar oil, and a sprinkle of fresh thyme leaves. Serve immediately.