Heat oil in small pan and cook onion until just starting to brown. Add lentils and bay leaf. Stir to coat in oil. Add vegetable stock and stir. Bring to a boil, reduce heat, cover, and simmer 40 minutes or until liquid is absorbed and lentils are tender, but not mushy.
When lentils are cooked, remove from heat and discard bay leaf. Stir in spinach and red pepper. Season to taste and serve with fresh parsley, olive oil, and balsamic vinegar.