Ultimate Cheesy Shepherd's Pie Casserole

Ingredients

• 2 tablespoons olive oil (30 ml)
• 1 large onion, diced (about 1 cup, 150g)
• 2 carrots, diced (about 1 cup, 150g)
• 2 celery stalks, diced (about 1 cup, 150g)
• 1.5 lbs ground lamb or beef (680g)
• 2 cloves garlic, minced
• 1 teaspoon dried thyme
• 1 teaspoon dried rosemary
• 1/2 teaspoon salt
• 1/4 teaspoon black pepper
• 2 tablespoons all-purpose flour (25g)
• 1 cup beef broth (240ml)
• 1/2 cup frozen peas (80g)
• 2 tablespoons Worcestershire sauce (30ml)
• 1 tablespoon tomato paste (15g)
• 3 lbs russet potatoes, peeled and quartered (1.36kg)
• 1/2 cup milk (120ml)
• 4 tablespoons unsalted butter (57g)
• 1/2 cup sour cream (120g)
• 1 cup shredded cheddar cheese (113g), divided
• 1/4 teaspoon salt
• 1/8 teaspoon black pepper