Preheat the oven to 200°C. Cook the quinoa until tender in lightly salted water for fifteen minutes, drain and let it rest, covered. Remove the stems from the portobellos and brush the caps with half of the olive oil, salt and pepper.
Heat the remaining oil in a skillet, briefly cook the finely minced garlic and let the spinach wilt in half a minute. Squeeze out any excess moisture, otherwise the mushrooms will become mushy. Mix the spinach with the cooked quinoa and some thyme.
Fill the portobellos with the spinach-quinoa mixture and lay halloumi slices on top. Bake for fifteen minutes in the oven until the cheese is golden brown and lightly bubbling. Sprinkle the toasted pine nuts on top and serve. Halloumi provides plenty of protein and stays firm when heated, making this vegetarian dinner just as satisfying as a meat dish.