Mince the garlic and dice the onion. Cook both in a large pot with olive oil over medium heat until tender. Meanwhile, slice the celery and peel and slice the carrots. Add the celery and carrots to the pot and continue to sauté for about 5 more minutes.
Drain the can of black beans and add it to the pot along with the dry lentils, cumin, oregano, smoked paprika, cayenne pepper, and some freshly cracked pepper (10-15 cranks of a pepper mill). Finally, add the diced tomatoes (with juices) and vegetable broth, and stir to combine.
Increase the heat to medium high and allow the pot to come up to a boil. Once it reaches a boil, turn the heat down to low, place a lid on top, and let it simmer for 30 minutes. After 30 minutes, test the lentils to make sure they are tender. If not, continue to simmer until they are tender.**
Taste the soup and add salt as needed (I added ½ tsp). Serve hot.
*I use Better Than Bouillon brand soup base to make my vegetable broth. It's less expensive than canned or boxed broths.
**Brown lentils take about 20-30 minutes to cook, green or French lentils take closer to 45 minutes to cook.