Heat the olive oil in a large pot over medium heat. Add the onion and garlic and cook for 3 minutes until the onion turns soft.
Stir in the carrots and cook for another 4 minutes to start softening them.
Add the harissa paste and stir for 1 minute to coat the vegetables.
Pour in the diced tomatoes, vegetable broth, and lentils. Bring the mixture to a boil.
Reduce the heat to low, cover the pot, and simmer for 20 minutes.
Add the zucchini, then simmer uncovered for 10 more minutes until the lentils are tender and the zucchini is cooked through.
Season with salt and black pepper. Ladle into bowls and top with fresh parsley.