Preheat oven to 375˚F
Clean, peel, and chop potatoes, place in large pot with ½ cup of beef stock, and ½ teaspoon salt. Bring to a boil, reduce heat and cover. Cook until fork tender, approximately 20 minutes. Work on step 3 while potatoes are cooking.
Heat 1 tablespoon olive oil in a cast iron skillet set on medium high heat. Once the oil begins to smoke, place 1 inch pieces of ground meat in the pan leaving space between. Sear for two minutes then stir the meat and continue to cook until browned. Cook in batches if necessary to ensure not to crowd the pan. Keep cooked meat aside in a bowl.
Using the same pan, add an additional 1 tablespoon of oil (if necessary). Sauté the onions for 30 seconds, then add mushrooms. Season with salt and pepper. Once mushrooms release liquid, add minced garlic, and continue to cook until mushrooms are tender. Mix the browned beef into the mushrooms, then add the carrots, peas, and corn. Pour in broth, tomato paste, worcestershire and cook until thickened, about 5-7 minutes. Taste, and adjust seasoning to taste.
In the pot with the potatoes, add 2 tablespoon of olive oil and black pepper. Mash until smooth and taste for seasoning. Adjust as necessary. Crack and egg open and immediately mash it into the mixture as quick as possible so the egg does not scramble.
Smear potatoes on top of the meat mixture and smooth or distribute evenly to achieve your desired look
Cook in preheated oven for 50 mins or until potatoes are golden brown