Put lentils in pot and pour water over them until an inch above the lentils. Bring to boil and then let simmer with the lid on, about 20-25 minutes. The lentils should be tender but not mushy. Add more water if needed. Drain and add salt.
Meanwhile, bake squash halves, cut side down, on a spray-oiled cookie sheet at 375° for 30 minutes or until flesh is soft.
Heat olive oil and sauté onions and mushrooms until onions are transparent, about 8-10 minutes.
Transfer cooked lentils and onion mixture to a large bowl. Use a spoon to scrape flesh from squash leaving skin intact and add to lentils and onions. Add remaining ingredients and mix thoroughly.
Spoon mixture into squash shells and reheat just before serving.
