Season the ribs with salt and pepper. In a large cast iron pan brown the ribs for 3 to 5 minutes per side. Stack them inside the slow cooker.
In the same pan, add the onion and sauté 5 to 10 minutes until golden and soft. Add the carrots, celery and garlic and cook an additional 5 minutes. Add the spices and as much of the broth that will fit, bring to a simmer then pour the entire contents into the slow cooker, along with the can of Guinness.
Turn the slow cooker to LOW and let braise for 8 hours (or so). Once the ribs are nicely done, remove them and set inside a serving dish. Pour the liquid into a pot and add the cornstarch. Simmer until thickened to your liking and pour over the ribs. Serve.
DUTCH OVEN METHOD: Follow the same method as above, but reduce the amount of ingredients to half so it will fit. Do the sautéing and braising in the same big pot. Place in the oven for 2 or 3 hours at 350°F.
PRESSURE COOKER METHOD: Follow the browning method but reduce the amount of ingredients to half so it all fits. Bring to full pressure and cook for 30 minutes. Let pressure release naturally, which should take between 15 to 20 minutes.