Heat 1 tablespoon olive oil in a large saucepan over medium heat. Stir in onion and salt; cook until softened and translucent, about 4 minutes. Stir in lentils and cook for 1 minute. Pour in water, increase the heat to high, and bring to a boil. Reduce the heat to medium-low, cover, and simmer until lentils are tender, about 15 minutes.

While the lentils are cooking, heat the remaining 2 tablespoons olive oil in a large skillet over medium heat. Add collard greens and cook until wilted, about 10 minutes.

When lentils are tender, stir in collard greens, garlic, cumin, and cinnamon; simmer for 10 minutes.