Cook Mirepoix: Heat the vegetable oil in a large Dutch oven or soup pot over medium-high heat. Add the chopped onion, carrots and celery then season with salt and pepper. Saute for about 5 minutes until the onion is soft and translucent.
Add Potatoes: Add the potatoes to the pot and stir. Cook for 2 more minutes.
Cook: Add the fire roasted tomatoes and chicken broth to the pot, stir then bring to a boil. Turn down the heat to a medium-low and cook for about 15 minutes or until the potatoes are fork tender.
Finish: Stir in the shredded chicken and lemon juice. Taste for seasoning and adjust with salt and pepper as necessary.
Serve: Garnish with parsley. Serve while warm.