Preheat oven to 375 degrees.
Cut potatoes into large piece. Put in pot with cold water (to cover) and salt. Over high heat, cook until potatoes are soft.
Once potatoes are done, drain and using a stand mixer, add potatoes to bowl. In a small pot, add heavy cream, 4 tablespoons of butter and milk. Bring to a boil, then remove from heat and add to bowl with potatoes. Add white pepper. Slowly mix until potatoes are smooth.
*Note: you may not need all liquid, so add in increments until potatoes are fluffy and not too soft.
In a pan, brown meat. Drain off fat. Set aside.
In a new pan, melt 2 tablespoons of butter over medium heat. Add mushrooms and sprinkle of salt. Cook until browned, about 7 minutes. Add carrots and cook until slightly softened, about 5 minutes.
Turn heat to medium low and add tomato paste. Stir really well and cook until paste has browned, about 5 minutes. Be sure to continuously stir to prevent paste from burning.
Add garlic and cook for 1 minute. Slowly add wine and stir well. Be sure to scrape bottom to get all browned bits up! Cook until 75% of the wine is cooked off and remove from heat.
Add peas, corn, soy sauce and 1 1/2 cups stock to the pot. In a small bowl, mix remaining stock and cornstarch together until smooth.
Add pot back to medium heat and slowly add cornstarch mixture. Bring to boil, stir while mixture thickens and then remove from heat. Add cooked beef.
In a baking dish, add meat mixture and spread evenly. Spoon on mashed potatoes and gently spread. Bake for 30 minutes or until meat mixture begins to bubble. Let cool for 10 minutes and enjoy!