Heat the olive oil in a large pot over medium heat. Add the diced onion and cook until softened.
Stir in the minced garlic and cumin. Cook for one minute until fragrant.
Add the rinsed lentils and vegetable broth. Bring to a boil, then reduce to a simmer. Cover and cook for 25 minutes.
Stir in the chopped Swiss chard. Continue simmering uncovered for 10 minutes until the chard wilts and the lentils are tender.
Remove the pot from the heat. Stir in the lemon juice, lemon zest, salt, and black pepper.
Ladle into bowls and sprinkle with chopped fresh parsley before serving.