Preparation

Preheat the oven to 350°F. Coat a 6-cup ovenproof casserole with cooking spray and set aside. In a small pot, combine the potatoes with enough water to cover by 3". Bring to a boil and cook for 10 to 12 minutes, or until tender. Drain the potatoes, return to the pot, and add the milk, butter, 1/4 teaspoon of the salt, and 1/8 teaspoon of the pepper. Mash until smooth and set aside. Heat a nonstick skillet over medium-high heat. Add the sirloin and cook for 5 to 6 minutes, stirring occasionally, until browned. Drain the fat from the pan and transfer the beef to a bowl. Return the skillet to the heat and add the onion, peas and carrots, and thyme. Cook, stirring occasionally, for to 7 minutes, or until the vegetables are soft. Return the beef to the skillet and stir in the broth, wine, and tomato paste. Cook for 2 to 3 minutes, or until the liquid is almost evaporated. Stir in the remaining 1/2 teaspoon salt and 1/8 teaspoon pepper. Transfer the beef mixture to the prepared casserole dish. Spread the reserved sweet potatoes evenly over the top of the beef. Bake for 25 to 30 minutes, or until the potatoes are lightly browned.