Heat butter in a large ovenproof pot or casserole dish. Gently cook the onion, carrots, celery, fennel, mushrooms, garlic, thyme and rosemary with a large pinch of salt until they are very soft, about 10-15 minutes.
If using lamb mince stir it in now and cook until completely brown. If using leftover roast lamb add it to the pan and just warm it for a couple of minutes.
Then add the tomatoes, tomato paste and broth. Simmer, stirring occasionally, for 15 minutes until slightly thickened.
In the meantime, boil or steam the sweet potatoes until tender, approximately 12 minutes. Drain well and then mash with the cream or milk, butter, salt and pepper until very smooth.
Season the lamb mixture to taste with salt and pepper and then spoon the mashed sweet potatoes evenly over the top.
Place in the oven for 20 minutes or until golden and bubbling.
Serve with salad or steamed veggies drizzled with olive oil.
Instead of lamb this pie is very delicious made with leftover roast chicken.
Try replacing the sweet potato mash with pureed cauliflower.
Instead of the fresh herbs you could use a dried herb mix.
Replace the lamb with beef to make a Cottage Pie.
Vary the flavours by adding anchovy fillets, cumin, cinnamon, red or white wine or Worcestershire sauce.