Heat olive oil in a large pot over medium heat. Add diced onion and chopped carrots, sautéing until softened, about 5-7 minutes.
Add minced garlic and cook for another 1-2 minutes until fragrant.
Pour in the lentils, diced tomatoes with juices, vegetable broth, and water. Stir to combine.
Bring the soup to a boil, then reduce heat to low and simmer, covered, for about 30 minutes or until lentils are tender.
Stir in the chopped collard greens and continue cooking uncovered for 10 minutes until greens are wilted and tender.
Season with salt and pepper to taste.
Remove the soup from heat and stir in the fresh lemon juice to brighten the flavors.
Ladle into bowls and serve warm, optionally with crusty bread.