Preheat the oven to 400ºF. Prepare a large baking sheet by lightly spritzing with non-stick spray or covering with foil.
In a small dish, stir together the harissa, olive oil, honey, salt, and lemon juice. Set the marinade aside.
Wash the potatoes well and cut into one-inch cubes. Wash and peel the carrots, then slice into one-inch sections. Cut the broccoli into florets, and chop the onion into large sections. Place the potatoes, carrots, broccoli, and onion on the prepared baking sheet.
Pour the harissa marinade over the vegetables and use your hands to toss the vegetables until they are evenly coated in the marinade.
Roast the vegetables in the preheated oven for 30 minutes, stir, and then continue roasting for another 10 minutes, or until the vegetables are tender and the edges have achieved a nice deep brown color. Top with chopped cilantro just before serving.