• Prepare the mashed potatoes if you are not using store-bought. Keep warm and set aside. If using store-bought, remove from the container and heat until warm.
• If making your own, peel the potatoes and cut into 1-inch inch cubes. In a pot of boiling water, add the potatoes and cook for 20 minutes or so, until tender with a fork.
• Drain the potatoes and return to the pot; mash with a potato masher and add the butter and milk; season with salt and pepper to taste. Mix together until smooth and set aside.

Make the meat mixture:

• In a large skillet, brown the ground beef until cooked through; drain off any fat.
• Add the onion, carrots, and celery, and cook until softened, approximately 5 minutes.
• Next, add tomato paste, garlic, thyme, cornstarch (or flour), and celery salt. Cook for one minute. Mix well and pour in the Guinness beer and add the Worcestershire sauce.
• Stir and cover the skillet with a lid, reduce the heat to low, and simmer for 10-15 minutes.
• Preheat the oven to 375 F.
• After simmering, stir the peas into the beef mixture. Spoon the mixture into an 8x8 baking dish (approximately 8×8 inches).
• Spread the mashed potatoes on top, to completely cover the meat mixture. If you prefer, you can take a fork and make a fun design on top of the mashed potatoes. They will be stacked high.
• Sprinkle lightly with a small amount of paprika.
• Bake until the top of the potatoes start to brown, and the meat mixture is heated through, for 20 minutes.  Remove from the oven and add a sprinkle of fresh chopped parsley.
• Serve warm.