1. Heat oven to 375 degrees. Add potatoes to a large lidded pot and cover with 1 inch of cold water. Bring to a boil; reduce heat to a simmer and cook, covered, 10 minutes or until fork-tender. Drain and return to pot. Add milk, 2 tbsp of the butter, 1/2 tsp of the salt and 1/4 tsp of the pepper. Mash until smooth. Cover and set aside.
2. Meanwhile, add remaining 1 tbsp butter to a large saute pan over medium heat. Stir in beef, breaking apart with a spoon, as well as carrots, celery and onion. Cook 15 minutes until vegetables are softened. Stir in flour; cook 1 minute. Stir in tomato paste, beef broth and peas. Bring to a simmer and cook 3 minutes, until liquid is slightly thickened and peas are thawed. Stir in remaining 1/2 tsp of the salt, remaining 1/4 tsp of the pepper, the thyme and rosemary.
3. Transfer meat-and-vegetable mixture to a 13 x 9 x 2-inch baking dish. Spread sweet potatoes over mixture, leaving a 1-inch border around the edge. Swirl top with back of a spoon, if desired. Bake at 375 degrees for 30 to 35 minutes or until shepherd's pie is bubbling.