Heat the olive oil in a large pot over medium heat. Add the diced onion and cook until it softens, about 5 minutes. Stir in the minced garlic and cook for 1 minute more.
Add the cumin, coriander, and harissa paste. Stir for 30 seconds until the spices smell fragrant.
Pour in the rinsed lentils, cubed potatoes, diced tomatoes with their juices, and vegetable broth. Bring the mixture to a boil.
Reduce the heat to low, cover the pot, and simmer for 25 to 30 minutes until the lentils and potatoes are tender. Stir occasionally to prevent sticking.
Taste and season with salt and black pepper. Ladle into bowls and top each serving with chopped parsley. Serve lemon wedges on the side.