Tested size: 4 servings; makes about 8 cups
Place the lentils in a bowl and cover with water; let sit while you prep and start to cook the vegetables.
Cut the zucchini, if using, and celery into ½-inch dice. Scrub the carrot and peel the onion, then cut into same-size dice as the zucchini and celery. Peel the potato and cut into bite-size chunks. Pour the can of tomatoes into a bowl (with the juice), then coarsely chop in the bowl.
Heat the oil in a large pot over medium-high heat. Once the oil shimmers, stir in the zucchini, if using, the celery, carrot, onion and potato. Cook for about 5 minutes, stirring a few times, until the onion has softened. Drain the lentils, then add to the pot, along with the ground spices and cayenne, if using. Pour in the tomatoes and their juice and the 2 1/2 cups of water, plus a generous pinch of salt and pepper; once the mixture comes to a boil, cover and cook for 5 minutes.
While the stew cooks, finely chop the parsley leaves. Cut the lime into wedges.
Reduce the heat to medium-low, cook for 10 to 15 minutes, stirring a few times, until all the vegetables are tender. Taste, and adjust the salt and/or pepper as needed.
Top each portion with parsley and serve hot, with lime wedges.