Preheat the oven to 425 degrees F. Trim the fat off the corned beef and cut it into 3/4 inch chunks. Toss the corned beef with the flour to coat the chunks on all sides. Reserve the seasoning packet.
Place a large sauce pot over medium heat. Add 4 tablespoon butter to the sauce pot. Once melted, add in the onions. Sauté the onions for 3 minutes to soften. Then add in the corned beef and brown the chunks on all sides.
While the meat is browning, add in the corned beef seasoning packet, dried thyme, garlic powder, 1 teaspoon salt, and 1/2 teaspoon ground black pepper. Stir well.
Next add in the sliced carrots, corn, and peas. Then stir in the stout beer and cream. Stir and simmer until the liquid thickens into gravy. Pour the mixture into a 3-quart baking dish.
Use a mandolin slicer to slice the potatoes into 1/8 inch rounds. You can use a chef's knife to do this, but it's much easier to keep them all the same thickness with a mandolin slicer.
Melt the remaining 3 tablespoons butter. Toss the potato slices in the butter to keep them from browning.
Starting on the outer edge, place the potato slices on the beef mixture, overlapping the potatoes as you go. You can make any pattern you like. Overlap the lines as needed, until you reach the center.
Salt and pepper the top of the potatoes liberally. Then cover the top tightly with foil. Bake for 50 minutes covered.
Then remove the foil and bake another 20 minutes, until the potatoes are brown and crispy. *If the potatoes are still white, you can turn the broiler on for 1-5 minutes to brown the top, but watch it closely so it doesn't burn!