Preheat oven to 350ºF. Spray a 1-1/2 quart baking dish with nonstick cooking spray.
Place potatoes in large saucepan; add cold water to cover by 2". Bring to a boil over medium-high heat; cook 16 to 18 minutes or until tender. Drain potatoes; return to saucepan.
In a small saucepan, heat milk over medium-high heat until hot. Add 3 Tablespoons butter, 1/2 teaspoon salt and 1/4 teaspoon pepper; cook until butter is melted. Pour milk mixture into saucepan with potatoes; mash until smooth. Set aside.
Melt remaining 2 Tablespoons butter in a large nonstick skillet over medium heat. Add onions, carrots and thyme; cook 8 to 10 minutes or until vegetables are softened but not browned, stirring occasionally. Add lamb; cook over medium-high heat 4 minutes or until no longer pink. Drain excess fat. Return skillet to heat; cook lamb 5 to 6 additional minutes or until lamb is lightly browned. Add tomato paste and Worcestershire sauce; cook 1 minute. Stir in broth; bring to a boil and cook 7 to 8 minutes or until nearly evaporated. Stir in peas, remaining 1/2 teaspoon salt and 1/4 teaspoon pepper; cook 1 minute. Transfer mixture to prepared baking dish.
Spread mashed potatoes in an even layer over the lamb mixture; use spatula to swirl potatoes or fork to make crosshatch design on top.
Bake about 35 minutes or until filling is hot and bubbly and potatoes begin to brown.