Heat the olive oil in a large pot over medium heat. Add the onion and cook until it softens, about 5 minutes. Stir in the garlic along with the cumin, coriander, paprika, cinnamon, turmeric, and cayenne. Cook for 1 minute until the spices smell fragrant.
Add the carrots and red bell pepper. Stir to coat them in the spices, then pour in the diced tomatoes and vegetable broth. Bring the mixture to a simmer, cover, and cook for 15 minutes.
Stir in the zucchini and chickpeas. Continue simmering uncovered for another 10 minutes until the vegetables are tender but not mushy. Season with salt to taste.
While the tagine simmers, bring the water to a boil in a small saucepan. Stir in the couscous, cover, and remove from heat. Let it stand for 5 minutes, then fluff with a fork.
Spoon the couscous into bowls and top with the tagine. Finish each serving with fresh cilantro and a squeeze of lemon.