Foolproof Savory Shepherd'S Pie

Ingredients

• 4 ounces button mushrooms
• 1/2 yellow onion
• 1 stalk celery
• 1 carrots
• 1 tablespoon butter
• 1 tablespoon olive oil
• 1 teaspoon salt divided
• 1/4 teaspoon black pepper
• 1 1/2 pounds lean ground beef
• 1 teaspoon dried thyme
• 1 dash nutmeg
• 1 teaspoon worchestershire
• 1 tablespoon tomato paste
• 1/3 cup frozen peas
• 1/3 cup frozen corn
• 2 tablespoons all-purpose flour
• 1 cup beef broth
• 2 pounds yukon gold potatoes
• 1/4 cup unsalted butter
• 1/4 cup sour cream
• 1 teaspoon salt
• black pepper
• 2 tablespoons milk