Prepare your potatoes. Peel the potatoes and cut into 1 1/2 inch cubes. Cook in boiling water until tender, approximately 20-25 minutes.
Drain the potatoes and return to the pot. Mash with the butter and milk, and season with salt and pepper to taste. Set aside when done.
While the potatoes are cooking, start your filling. Brown the ground beef in a medium to large pot over medium heat until cooked through.
Add the carrots and onion, and cook until softened, approximately 5 minutes.
Add the tomato paste, garlic, cornstarch (or flour), and 1/4 teaspoon salt. Cook for one minute.
Add the Guinness and Worcestershire to the meat, scraping up any browned bits from the bottom of the pot.
Cover, reduce heat to low, and simmer 10-15 minutes.
Preheat oven to 375F.
Stir the peas into the beef mixture. Pour the beef mixture into a small baking dish (approximately 8x8 inches).
Spread the potatoes across the top of the meat in the dish, edge to edge, completely sealing the meat in to trap the moisture and juices.
Bake until the top is starting to brown, approximately 20 minutes. Serve warm.
If you are trying to prevent sauce from pooling on top, make an ever so subtle mound in the center of the dish with the potatoes and have the potatoes very slightly slope downward toward the edges and corners. This should help encourage the sauce to go back in the bottom instead of sitting on top.