Heat the olive oil in a large pot over medium heat. Add the diced onion and cook until it softens, about 5 minutes.
Stir in the minced garlic and berbere spice. Cook for 1 minute until the spices smell fragrant.
Add the tomato paste and stir to coat the onion mixture. Cook for another minute.
Add the cubed beets, sliced carrots, and rinsed lentils to the pot. Pour in the vegetable broth and add the salt.
Bring the mixture to a boil, then reduce the heat to low. Cover and simmer for 30 to 35 minutes, stirring occasionally, until the lentils and beets are tender.
Taste and adjust salt if needed. Serve the stew in bowls with chopped cilantro and lemon wedges on the side.