Prepare the Halloween vegetables: Peel the potatoes and cut each potato in half crosswise. Stand each half upright and carefully slice it vertically into 3–4 slices, about ½ inch thick. Use a small, sharp paring knife to cut the bottom of each slice into a wavy ghost shape. You can also use the bottom edge of a ghost cookie cutter to help shape the potatoes. Use a sturdy straw or small cookie cutter to make two eyes and a mouth in each ghost. Place the finished potato ghosts in a bowl of cold water to keep them from turning brown. Set aside while you prepare the other vegetables. Peel the carrots and trim off the ends. Cut a small stem shape into the top of each carrot by making two straight cuts. Carefully cut two small triangles from either side of the stem and remove them to create the pumpkin shape. Slice the carrots into rounds. Each slice will look like a little pumpkin!
Clean the mushrooms and cut them in half. If they're large, cut them into thick slices instead. Use a sturdy drinking straw to poke two holes near the top of each mushroom half for the eyes. Use a small, sharp paring knife to make a few short vertical cuts along the bottom to create the teeth. Set the finished mushroom skulls aside until you're ready to cook them.
Coat the beef. In a large bowl or zip-top bag, combine the flour, garlic powder, 1 teaspoon salt, and ½ teaspoon black pepper. Add the beef and toss until evenly coated. Shake off any excess flour.
Brown the beef. Heat 2 tablespoons olive oil in a large Dutch oven over medium-high heat. Brown the beef in batches for 2–3 minutes per side. Transfer to a plate and set aside.
Sauté the vegetables. Add the remaining 1 tablespoon olive oil to the pot. Add the onion and celery and cook for about 5 minutes, until softened.
Add the flavorings. Stir in the tomato paste, minced garlic, and Worcestershire sauce. Cook for 1 minute, stirring frequently.
Simmer the beef. Add the beef broth, thyme, and bay leaves. Return the browned beef and any accumulated juices to the pot. Scrape up the browned bits from the bottom. Bring to a boil, then reduce the heat to low or medium-low. Cover and simmer gently for 1–1½ hours, or until the beef is becoming tender.
Add the Halloween potatoes and carrots. Drain the potatoes and add them to the stew along with the pumpkin carrots. Cover and simmer for about 15–20 minutes, or until the vegetables are fork-tender. Stir gently to keep the shapes intact. If the beef needs more time, continue simmering until tender.
Cook the skull mushrooms. For the best results, melt 1 tablespoon butter in a skillet over medium-high heat. Sauté the mushrooms until golden brown on both sides. Gently stir them into the stew once the potatoes and carrots are tender. Alternatively, add the mushrooms to the stew with the potatoes and carrots.
Season and serve. Remove the bay leaves. Taste and add more salt and pepper if needed. Ladle into bowls and serve warm!
