Heat the olive oil in a large pot over medium heat. Add the onion and garlic and cook for 4 minutes until the onion softens.
Add the mushrooms and cook for 5 minutes until they release their liquid and start to brown.
Stir in the lentils, harissa paste, diced tomatoes, vegetable broth, cumin, salt, and pepper.
Bring the mixture to a boil, then lower the heat and cover the pot.
Simmer for 30 minutes, stirring now and then, until the lentils are tender.
Taste and adjust salt or harissa if needed. Ladle into bowls and top with parsley. Serve with lemon wedges on the side.