Heat olive oil in a skillet over medium-high heat. Add onions, carrots and celery. Cook until softened, about 5-6 minutes.
Add garlic and cook until fragrant, stirring occasionally, about 30 seconds. Transfer veggies out of pan and add to slow cooker.
Add remaining ingredients to slow cooker and stir to combine. Cook for 3-4 hours on high or 6-8 hours on low.
To serve, transfer chicken out of slow cooker and shred with two forks. Return to slow cooker and stir to combine.
