Nutrition: the stew alone is vegan. Depending on your choice of condiments, it's likely gluten free and vegetarian after adding sauces. Pesto will be the most likely culprit in spoiling either of these classifications, so be sure to check your ingredients in the case of allergies or strict adherence.
If you don't have harissa, I'd recommend adding chili powder (even better: ancho chili powder), ground cumin, ground coriander, ground caraway seeds, and a bit of cayenne pepper to the onions when they're almost softened. Stir for a couple minutes, until the spices are aromatic. You may also wish to add smaller quantities "raw" at the end, to finish the soup. In lieu of all that, sriracha will provide some heat and flavor, although it won't match the complexity of the harissa.
The lentils make this stew insanely filling, especially when served with bread. The recipe will likely feed 12 or more--just don't use giant soup bowls, or you may end up throwing a lot out. The recipe halves fairly well (you can still add the full can of tomatoes) and freezes great.
Feel free to make a day or two ahead--the flavors noticeably deepen after just 12 hours in the refrigerator.
Recipe adapted from Food & Wine magazine, December 2014 print issue.