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.
Add the garlic, sauté until transparent, around 4 minutes.
Add the diced carrots, celery, mushrooms and walnuts. Sauté for 5 minutes until tender.
Add the Italian seasoning, paprika, salt and red pepper flakes, stir to combine.
Pour in the crushed tomatoes and stir, add ½ cup water if the sauce is a little thick, bring to a gentle simmer and cook for 5 minutes.
Transfer the sauce to a food processor and pulse several times, making sure to keep the sauce slightly chunky.
Return the sauce to the pan along with the cooked lentils and ½ - 1 cup water or vegetable stock.
Bring to a simmer and cook for 5-10 minutes or until the sauce begins to thicken.
Season to taste.
Serve warm with your favorite pasta.