For the ground lamb filling

• 1 1/2 lbs (24oz) ground lamb
• 2 Tbsp olive oil
• 1 1/2 tsp kosher salt, divided
• 3 large carrots, cut into 1/4-inch dice
• 2 medium (or 1 large) sweet onions, cut into 1/4-inch dice
• 3 stalks celery, cut into 1/4-inch dice
• 4 sprigs fresh thyme
• 1/4 tsp freshly ground black pepper
• 2 Tbsp tomato paste
• 1/2 cup (118ml) red wine
• 2 tbsp Worcestershire sauce
• 1 Tbsp Dijon mustard
• 1 1/2 cups (355ml) low-sodium beef broth, chicken broth or vegetable broth
• 2 dried bay leaves
• 2 cups frozen English peas

For the root vegetable topping

• 1 large potato, cut into 1/2-inch cubes (Russet or Yukon gold, preferably)
• 1 large sweet potato, peeled and cut into 1/2-inch cubes
• 1 large parsnip, peeled and cut into 1/2-inch cubes
• 4 Tbsp unsalted butter
• 1/3 cup (79ml) milk, any kind
• 1 cup (4oz) sharp white Cheddar cheese, shredded
• 1 tsp kosher salt