Place potatoes in a large saucepan and cover with water. Bring to a boil. Reduce heat; cover and cook for 10-15 minutes or until tender. Drain and Mash potatoes, gradually adding butter and enough milk to reach desired consistency. Stir in ½ teaspoon salt and ¼ teaspoon pepper.
Meanwhile, in a large skillet, cook the beef, onion, and garlic over medium heat until meat is no longer pink, drain.
Add tomato paste; cook for 2 minutes. Add the mushrooms, carrots, broth, wine, Worcestershire sauce and thyme. Bring to a boil. Reduce heat; simmer, uncovered, until most of the liquid is evaporated. Stir in peas. Season with remaining salt and pepper.
Transfer beef mixture to a greased slow cooker. Spread mashed potatoes over top. Cover and cook on low for 5-6 hours or until bubbly. Sprinkle with cheese. Cover and cook 10 minutes longer or until cheese is melted. Just before serving, sprinkle with parsley if using and enjoy!