Lamb Filling: Preheat oven to 400 degrees F (204 degrees C). Meanwhile, heat oil in ovenproof high-sided 10-inch skillet set over medium-high heat.
Cook, stirring often, for 8 to 10 minutes or until browned; drain fat well from pan.
Stir in onion, carrot, celery, red pepper and garlic. Cook, stirring occasionally, for 5 to 8 minutes or until vegetables are tender. Pour in Patak's Tikka Masala Sauce; stirring to mix well; bring to a simmer. Simmer for 5 minutes.
Sweet Potato Topping: Cook sweet potatoes and Yukon Gold potatoes in boiling salted water set over medium heat for 18 to 20 minutes or until tender.
Drain well. Return back to pot. Add milk, butter, salt and pepper; mash until smooth.
Spoon Sweet Potato Topping over filling; smooth top. Place skillet on a baking sheet.
Bake for 15 to 20 minutes or until filling is bubbling and topping is hot and lightly golden. Garnish with cilantro.