Preheat oven to 325°F (163°C). Heat olive oil in a stovetop-safe casserole dish or braising pan over medium heat.
Season the stew meat with salt and pepper, then sear in the hot oil until browned on all sides. Remove and set aside.
In the same pan, sauté onions until translucent, about 5 minutes. Add garlic and cook for an additional minute.
Stir in tomato paste, cooking for 1 minute. Sprinkle with flour and stir for another minute to remove the raw flour taste.
Pour in Guinness beer, scraping the bottom of the pan to deglaze and enhance flavor.
Add beef broth, Worcestershire sauce, and thyme. Return the seared meat to the pan, cover, and transfer to the oven. Bake for 1½ hours.
Stir in carrots, corn, and peas, then continue baking, uncovered, for another 30 minutes until vegetables are tender.
Remove from the oven and let the stew rest for a few minutes.
Spread mashed potatoes over the top, then broil for 3-5 minutes until golden brown.
Serve hot and enjoy!