Heat a dutch oven on the stovetop over medium heat. Increase to medium-high heat, then add 2 tablespoons of olive oil. Season the meat with 2 teaspoons of kosher salt, and ½ teaspoon of black pepper. Add to the hot pot, then cook in batches, browning on at least two sides to lock in the moisture, then transfer meat to the slow cooker and set aside. Do not clean the Dutch oven.
Lower the heat to medium, then add in butter to melt, followed by the onions, with a good pinch of salt and pepper. Cook onions until softened, translucent, and starting to turn golden brown, about 5-7 minutes.
Add flour, garlic, tomato paste, oregano, thyme, and smoked paprika to the dutch oven. Cook for 2-3 minutes until the tomato paste deepens in the color. Deglaze with the Guinness (or beef stock) and cook for a few minutes to cook off the alcohol in the beer.
Pour the onion and beer mixture over the beef in the slow cooker, followed by the beef stock, and Worcestershire sauce. Add bay leaves and thyme sprigs. Cook on high for 1.5 hours.
After 1 hour, add the potatoes, carrots, and parsnips, with a big pinch of salt and pepper. Cook for an additional 2.5 hours.
At hour 3, add in the optional peas, and continue cooking for 1 hour. Finally, remove the bay leaves, and thyme sprigs. Adjust the seasonings to taste.
Ladle into bowls. Garnish with fresh parsley, thyme, salt, and pepper, then serve with a hearty bread like a baguette, Irish soda bread, or rye bread. Store leftovers in the fridge for up to 5 days. Reheat gently on the stovetop or in the microwave. To freeze, allow leftovers to cool completely. Transfer to a freezer-safe container and store for up to 3 months. Defrost in the fridge overnight, and reheat in the microwave or on the stovetop.