Heat the teaspoon of olive oil in a large pot over medium heat. Once the oil is shimmering, add the diced onion, chopped celery, diced carrots, and chopped bell pepper. Sauté the vegetables for about 5-7 minutes, stirring occasionally, until they start to soften and the onions become translucent.

Add the tomato paste to the pot and stir it into the vegetables. Cook for another 1-2 minutes, allowing the tomato paste to coat the vegetables and start to caramelize slightly.

Pour in the can of diced tomatoes, including their juice. Stir to combine with the vegetables and tomato paste, scraping any browned bits from the bottom of the pot.

Add the dried lentils and cabbage to the pot and stir to combine with the vegetable mixture.

Pour in the broth and bring the mixture to a boil. Once boiling, reduce the heat to low, cover the pot, and let the soup simmer for about 20 minutes. This allows the lentils to start cooking and softening.

Taste the soup and season with salt and pepper as needed. Remember that the amount of salt you'll need can vary depending on the sodium content of your broth and canned tomatoes.

Once the flavors are to your liking and all vegetables are tender, your Lentil Cabbage Soup is ready to serve. Ladle it into bowls and enjoy while hot.