Heat olive oil in a frying pan over medium-high heat. Add onion and celery and cook, stirring, until onion is translucent, about 3 minutes. Add tomato paste and stir to combine.
Transfer onion mixture to a slow cooker. Add remaining stew ingredients and stir to combine. Cover and cook on low for 8 hours or on high for 4 hours.
To prepare the cheesy polenta place milk and water in a medium saucepan over medium-high heat. Bring mixture to the boil, then reduce temperature to medium. Gradually add polenta while whisking continuously.
Reduce heat to low, cover saucepan with lid and cook for 30 minutes or until mixture is smooth and creamy. You’ll need to whisk the mixture every 10 minutes to ensure polenta doesn’t stick to the bottom of the pan.
Remove saucepan from the heat. Stir through Parmesan and butter. Season to taste and serve immediately.