In a large skillet over medium-high heat, brown the ground lamb, breaking it up as it cooks. Drain off any excess fat and set the meat aside.

In the same skillet, add the chopped onion, carrots, and celery. Sauté for 5-7 minutes until the vegetables begin to soften. Add the minced garlic and cook for 1 minute more until fragrant.

Stir in the flour until absorbed, then add the tomato paste, Worcestershire sauce, beef broth, and dried thyme. Bring the mixture to a simmer, scraping up any browned bits from the bottom of the pan. Return the cooked lamb to the skillet and stir well to combine. Taste and adjust seasoning if necessary.

Transfer the lamb filling mixture to a 5-6 quart slow cooker, spreading it evenly across the bottom.

Prepare the mashed potatoes: Place the potatoes and a pinch of salt in a pot and cover with water. Boil until fork-tender (about 15-20 minutes). Drain thoroughly, return to the pot, and mash with butter, warm milk, salt, and pepper until smooth and creamy. Stir in the optional cheddar cheese.

Carefully spoon the mashed potatoes over the top of the meat filling in the slow cooker, spreading evenly to create a seal. Rough up the top surface with a fork to create peaks for better browning.

Cover and cook on LOW for 6 hours or on HIGH for 3 hours. For a slightly crispier top, switch the slow cooker to the 'Keep Warm' setting, remove the lid, and cook for another 15-20 minutes until the peaks start to lightly brown. Let rest for 10 minutes before serving.