Spread the cauliflower out on a large rimmed baking sheet. Drizzle with 3 tablespoons of the oil, ½ teaspoon salt and the cumin seeds. Roast 25 minutes, or until the cauliflower is tender and golden. Add the chickpeas and roast an additional 10 minutes.
Meanwhile, heat the remaining tablespoon oil in medium (8-10-inch) sauté pan. Add the onions and a pinch of salt. Cover the pan and cook over medium-low heat until the onions are very soft, 10-12 minutes. Stir in the garlic and harissa along with 2-3 tablespoons water and stir until blended and saucy. Pour over the cauliflower on the sheet pan.
Tear the parsley and cilantro into rough pieces and toss them over the chickpeas. Squeeze the lemon over, add the feta and gently mix together.
Transfer to a serving bowl and serve.
If you'd rather make this a vegan dish, simply omit the cheese.