Heat olive oil in a large pot over medium heat.
Sauté onion and garlic until soft and fragrant, about 3-4 minutes.
Add cubed beets, red bell pepper (if using), tomatoes, rinsed lentils, bay leaf, and stock or water.
Bring mixture to a boil, then lower heat to medium-low.
Simmer for 20–30 minutes until beets are tender and lentils are fully cooked.
For a smoother consistency, blend partially or mash with a potato masher (optional).
Remove bay leaf and season with salt and pepper to taste.
Serve hot with suggested garnishes.