Heat olive oil in a large heavy-bottomed pot over medium heat.
Add the diced onion, carrots, and celery; sauté for 8 to 10 minutes until softened and translucent.
Incorporate the mushrooms and red bell pepper, cooking for an additional 6 minutes until the mushrooms have released their moisture and started to brown.
Stir in the minced garlic and tomato paste, cooking for 2 minutes until the paste is well-distributed and fragrant.
Add the rinsed red lentils, crushed tomatoes, vegetable stock, and dried oregano to the pot.
Bring the mixture to a gentle boil, then reduce the heat to low and simmer, partially covered, for 25 to 30 minutes.
Stir occasionally to ensure the lentils do not stick to the bottom; cook until the lentils are tender and the sauce has reached a thick consistency.
Adjust seasoning with salt and black pepper; serve garnished with fresh basil over whole-wheat pasta or spiralized vegetables.