Cook your lentils according to package directions. Drain and set aside.
In a large pot, heat the olive oil over medium-high heat. Add the red onion and sauté until transparent, around 4 minutes. Add the garlic and cook until fragrant, or 1 more minute.
Add the carrots, celery, mushrooms, and walnuts, and sauté for 5 more minutes until tender.
Add in the garlic, Italian seasoning, paprika, salt, and red pepper flakes and stir to combine.
Pour the crushed tomatoes and stir. Bring to a gentle simmer. Cook for 5 more minutes and transfer to a food processor. Pulse until chunky.
Add the lentils back into the pan and warm through for 1-2 minutes. Pour the sauce and the seasonings and stir. Bring to a simmer and cook for 10 minutes until the sauce thickens a bit and the flavors combine.
Serve warm, with your favorite pasta.