Heat the vegetable oil in a large Dutch oven or heavy-bottomed pot over medium-high heat. Brown the beef cubes in batches, setting them aside as they brown.

Add the onion to the pot and sauté for 3 minutes until translucent. Stir in the garlic and cook for 1 minute more.

Return the browned beef to the pot. Add the carrots, parsnips, and potatoes. Stir in the tomato paste and cook for 2 minutes.

Pour in the beef stock and Guinness (if using). Add Worcestershire sauce, bay leaves, thyme, salt, and black pepper. Bring to a boil, then reduce heat to low, cover, and simmer for 1½ hours, stirring occasionally.

In a bowl, whisk together flour, baking powder, and salt. Rub in the cold butter with your fingertips until the mixture resembles coarse crumbs. Stir in the milk and parsley, mixing just until combined.

After 1½ hours, remove the lid from the stew. Drop tablespoonfuls of dumpling dough onto the surface of the simmering stew (about 12 dumplings).

Cover and cook for 25 minutes, or until the dumplings are puffed and cooked through. Discard the bay leaves, adjust seasoning if needed, and sprinkle with chopped parsley before serving.