1. Preheat oven to 400 degrees F.
2. In a large saucepot, cover potatoes with salted water by 1 inch; bring to a boil.
3. Reduce heat; simmer until fork-tender, 15 to 20 minutes. Drain.
4. Heat a large skillet over high heat. In two batches, cook ground beef until no longer pink, about 5 minutes per batch.
5. Transfer beef to a colander set in a bowl; let fat drain off and discard fat.
6. Add ¼ cup water to the skillet, scraping up browned bits (deglazing the fond) with a wooden spoon.
7. Reduce heat to medium; add onion and carrots. Cook, stirring occasionally, until softened, about 5 minutes.
8. Stir in tomato paste. Add flour; cook, stirring, 2 minutes.
9. Add Worcestershire sauce, 1 cup water, and beef. Season with 1 teaspoon salt and ¼ teaspoon pepper. Simmer until thickened, stirring occasionally, about 10 minutes.
10. Stir in peas; cook 1 minute.
11. Add filling to a 9-inch pie dish.
12. In large saucepot, bring milk and butter to a simmer; remove from heat.
13. Return potatoes to the saucepot and mash. Season with 2 teaspoons salt and ¼ teaspoon pepper.
14. Spread potato mixture over the pie; use a fork to make peaks.
15. Bake on a baking sheet until top is browned, 25 to 30 minutes. Cool slightly; serve.
Deglaze- to swirl or stir a liquid (usually wine or stock) in a pan to dissolve cooked food particles remaining on the bottom. The resulting mixture often becomes the base for a stock
Fond- French for “bottom”. Fond refers to the concentrated juices, drippings, and bits of food left in pans after foods are roasted or sautéed.