Heat olive oil in skillet over medium-high heat.
Add seitan chunks, searing 3-4 minutes until browned.
Transfer to slow cooker. In same skillet, sauté onion, carrot, celery 5 minutes until softened.
Add garlic, tomato paste, Italian seasoning, fennel.
Cook 1 minute until fragrant.
Deglaze with ½ cup broth, scraping brown bits.
Transfer to slow cooker.
Add lentils, crushed tomatoes, remaining broth, and soy sauce.
Stir. Cook LOW 6-8 hours or HIGH 3-4 hours until lentils tender.
Stir in nutritional yeast before serving.
Serve over cooked pasta, topped with ground flaxseed