Heat olive oil in a large skillet over medium heat. Add diced onion, carrot, and celery. Cook 7-10 minutes, stirring occasionally, until vegetables soften and onion turns translucent. Add minced garlic in the last 1-2 minutes.
1 onion, 1 carrot, 2 stalks celery, 3 cloves garlic, 2 tbsp olive oil
Push vegetables to the side of the pan. Add tomato paste to the center. Cook 2-3 minutes, stirring constantly, until it darkens to a deep red and smells sweet. Mix it into the vegetables.
2 tbsp tomato paste
Stir in rinsed lentils, crushed tomatoes, vegetable broth, oregano, and basil. Season with salt and pepper. Bring to a simmer.
1 cup brown lentils, 28 oz crushed tomatoes, 2 cups vegetable broth, 1 tsp dried oregano, 1 tsp dried basil
Reduce heat to medium-low. Let the sauce bubble gently for 30-40 minutes uncovered, stirring every 10 minutes. Lentils should be tender with a slight bite and the sauce thickened and clinging to a spoon.
Taste a lentil at 30 minutes; if still firm, simmer 5-10 minutes longer. Add salt, pepper, or a pinch of sugar if the sauce tastes too acidic. Stir in fresh basil or parsley if desired.
