For Instant Pot: Select SAUTE. Add olive oil, then Italian sausage. Cook until browned, drain fat. Add onion, celery, carrots, garlic. Sauté 3-5 minutes. Stir in tomato paste, Italian seasoning, red pepper flakes, salt. Cook 1 minute.
Add 1 cup rinsed lentils and 6 cups broth. Secure lid, select MANUAL/PRESSURE COOK, HIGH pressure for 15 minutes. Natural release 10 minutes, then quick release.
For Slow Cooker: Heat olive oil in a skillet. Brown Italian sausage, drain fat, transfer to slow cooker.
To slow cooker, add onion, celery, carrots, garlic, tomato paste, Italian seasoning, red pepper flakes, salt, 1 cup rinsed lentils, and 6 cups broth. Stir to combine.
Cover and cook on LOW for 6-8 hours or HIGH for 3-4 hours, until lentils are tender.
Taste and adjust seasonings as needed. Serve hot.