Ranchers Pie

Name: Rancher's Pie
Ingredients: For the mashed potatoes:; 2 pounds russet or Yukon Gold potatoes, peeled and cut into 1-inch pieces; 1/2 cup whole milk, warmed to 105°F; 4 tablespoons unsalted butter; Kosher salt, to taste, plus more for water; For the pie:; 4 tablespoons unsalted butter, divided; 2 medium carrots, peeled and diced; 2 stalks celery, diced; 1 small yellow onion, finely chopped; Kosher salt, to taste; Freshly cracked black pepper, to taste; 2 garlic cloves, peeled and minced; 2 pounds ground beef, preferably chuck or sirloin, or ground lamb; 3/4 to 1 cups low-sodium beef stock; 1/4 cup tomato paste; 3 tablespoons Worcestershire sauce; 1/2 cup frozen petite peas