Preheat oven to 350 degrees.
Peel sweet potatoes and cut into quarters. Boil until fork tender (about 15 minutes).
Drain the water and mash them, then place in a large bowl.
Add the butter and ½ cup milk to mashed sweet potatoes.
Stir until well combined and smooth.
Heat a large black iron skillet over medium heat. Add in the ground lamb and cook until browned (about 5 minutes).
Remove lamb from skillet and place it on a plate lined with papertowels to drain grease.
Add onions and mushrooms to skillet and saute until tender, approximately 5 minutes.
Return the lamb to the skillet with vegetables.
Add in peas, all spices, tomato paste and water. Bring to a simmer and cook for about 5 minutes or until tomato paste has melted and peas are cooked through.
Spoon mashed sweet potatoes over lamb mixture and smooth the top.
Place on a baking sheet and put on middle rack in the oven. Bake for 25 to 30 minutes at 350 degrees until edges begin to lightly brown and filling is bubbly.
Remove from oven and cool on wire rack for 10 minutes.
Sprinkle with chopped fresh parsley as a garnish if desired. Serve warm and enjoy!