Chop the onion, celery, carrot, and mushrooms. Mince the garlic. Heat the olive oil and ghee in the cooker over low heat. Add the onions and garlic and gently cook for 2-5 minutes or until the onion becomes clear, but not brown. Add the mushrooms, celery, and carrot and cook for another 5 minutes or until the mushrooms are lightly sautéed.
Add the lentils and broth. Lock the lid into place and cook at high pressure for 12 minutes. Allow the pressure to reduce gradually. When done, add the vinegar and salt to taste.