Heat the oil in a large Dutch oven over medium-high heat. Season the ribs with salt and pepper.
Add the ribs to the pan in batches and brown on all sides. Remove to a paper towel-lined plate and set aside.
Add the onions, carrots, celery, and garlic to the crock of a slow cooker. Season with salt and black pepper. Nestle the pork ribs into the vegetable mixture.
n a separate bowl, combine the tomato paste, vinegar, red pepper flakes, beer, and chicken stock. Pour the mixture over the pork ribs. Add the bay leaves.
Cook on high for 4 hours or on low for 6 hours.
Notes
Browning the ribs before slow cooking adds extra depth of flavor and is highly recommended.
Use a mild beer such as a lager or pale ale for best results. Avoid overly bitter beers like IPAs.
You can substitute the beer with additional chicken stock or apple juice if preferred.
Trim excess fat from the ribs if needed, but leave some for flavor.
For a thicker sauce, remove the ribs after cooking and simmer the liquid on the stovetop until reduced, or add a cornstarch slurry.
Leftovers store well and taste even better the next day as the flavors continue to develop.