Real deal lamb shepherd's pie

Bahan-bahan (4 porsi)

• 1.5 lb lamb (shoulder chop (bone in, see notes))
• 1 medium red onion (diced)
• 1 medium carrot (diced)
• 2 stalks celery (diced)
• 1 sprig fresh rosemary (leaves only, minced)
• 1 tbsp flour
• 3 large potatoes (Yukon gold preferred, peeled and cut into 1-2 inch chunks)
• 2 tbsp butter
• 1/4 cup milk
• 2 tbsp sour cream
• 2 tbsp shredded cheddar
• 1 -2 tbsp butter (to top, optional)