Heat the oil in a large heavy saucepan over medium heat. Add the onions, garlic and celery; cook for 5 minutes, stirring occasionally, until tender. Add the salt, pepper, cumin, turmeric, cinnamon and ginger. Cook for 2 minutes, stirring frequently. Add the chicken stock, lentils, tomato paste, sweet potatoes and tomatoes, breaking up the tomatoes with a spoon. Bring the mixture to a boil. Reduce the heat; cover and simmer for 40 minutes. Add the frozen peas; simmer for 5 minutes or until the peas are tender. Top each serving with a dollop of sour cream and a sprinkle of fresh parsley.