Preheat your oven to 400F. If you're cooking lentils from scratch, mix the dry lentils in a saucepan with enough water to cover them by two inches. Bring to a boil and reduce to a simmer. Simmer for 25 minutes, or until the lentils are still tender but retain their shape and firmness. You can start testing them at the 20 minute mark for doneness. Drain the lentils and set them aside.
While the lentils cook, toss the cauliflower florets and sliced onion with the oil, za'atar, lemon, paprika, salt, and black pepper. Spread the veggies onto two parchment-lined baking sheets. Roast the vegetables for 20-25 minutes, or until the cauliflower is tender and lightly browning, and the onions are getting crispy. Check on the veggies and stir them on the sheet halfway through roasting. Allow the roasted vegetables to cool to room temperature.
While the veggies roast, whisk together the tahini, water, garlic, lemon juice, salt, and harissa to make the dressing. If it's too thick, add a few extra tablespoons of water.
When the vegetables are cool, toss them together with the lentils and arugula. Serve with dressing immediately. Alternately, you can store the roasted veggies and lentils, arugula, and dressing separately and mix when ready to eat.