Preheat the oven to 200°C. Cut the cauliflower into small florets, slice the onion and mince the garlic cloves. Arrange the cauliflower on a baking sheet, drizzle with olive oil and toss well.
Slide the baking sheet into the oven and roast the cauliflower for 20 minutes until tender and light brown. Meanwhile, heat a little olive oil in a soup pan and sauté the onion and garlic until translucent. Add the vegetable broth, stir in the roasted cauliflower and let it simmer briefly.
Remove the pan from the heat and purée the ingredients with an immersion blender until smooth. Cut the halloumi into sturdy slices. Heat a dry grill pan or frying pan over medium-high heat and fry the halloumi slices on both sides until crispy and golden brown.
Ladle the warm cauliflower soup into bowls or deep plates. Distribute the grilled halloumi pieces over the portions. Finish the dish by sprinkling zaatar over the soup and squeezing fresh lemon juice over it to taste.