Preheat oven to 425°. Heat a large skillet over high heat. In two batches, cook lamb until no longer pink, about 5 minutes per batch. Transfer lamb to a colander set in a bowl; let fat drain off and discard.
Add ¼ cup water to the skillet, scraping up browned bits with a wooden spoon. Reduce heat to medium; add onion and carrots. Cook, stirring occasionally, until softened, about 5 minutes. Stir in tomato paste. Add flour; cook, stirring, 2 minutes.
Add Worcestershire sauce, 2 cups water, and lamb. Season with 2 teaspoons salt and ¼ teaspoon pepper. Simmer until thickened, stirring occasionally, about 10 minutes. Stir in peas; cook 1 minute. Divide among eight 8-ounce ramekins or two 9-inch glass pie dishes.
Meanwhile, make potato topping. Spread over pies; use a fork to make peaks. Bake on a baking sheet until tops are browned, 25 to 30 minutes. Cool slightly; serve.
In a medium saucepan, cover potatoes with salted water by 1 inch; bring to a boil. Reduce heat; simmer until fork-tender, 15 to 20 minutes. Drain.
In pan, bring milk and butter to a simmer; remove from heat. Return potatoes; mash. Season with 2 teaspoons salt and 1/4 teaspoon pepper.
I prepared my Shpard's pie in the crockpot. I placed the "soup" part in the bottom and the potatoes on top. I cooked the pie for 8 hours on low.