Take a medium saucepan and heat oil over medium heat, then add the onion and garlic and cook for about 4 to 5 minutes or until the onion has become tender and occasionally stir the mixture.
Take lentils, coriander, cumin, black pepper, and cinnamon and stir it into the mixture and cook the mixture for 2 minutes.
Take the chicken broth, celery, and sun-dried tomatoes and add these ingredients to the mixture and bring to a boil over high heat. Once mixture is brought to a boil reduce the heat to low and allow mixture to simmer covered for 25 minutes.
Take squash, bell pepper, and parsley and stir it into the mixture and continue to cook covered for 10 minutes or until the lentils have become tender.
Take plum tomatoes and cilantro and place over the soup right before serving.