SHEPHERD'S PIE

2 1/4 lb. red-skinned potatoes, scrubbed
1 1/2 cups warm milk
4 T butter
3 tsp kosher salt, divided
freshly ground black pepper
3 T olive oil, divided
1.5 lb ground beef (90% lean)
1 lb cremini mushrooms, quartered (about 4 cups)
1 tsp fresh thyme leaves
1 medium onion, finely chopped
2 ribs celery, finely chopped
2 cloves garlic, finely chopped
3 T tomato paste
1 cup low-sodium chicken stock
2 medium carrots, chopped
2 3/4 cups frozen peas
1 T Worcestershire sauce

Preheat oven to 425 degrees.  Cover potatoes with cold water in a large pot.  Bring to a boil, then simmer until tender when pierced with a fork; 20 to 25 minutes.  Drain and return to pot.  Add milk, butter, 1.5 tsp salt, and several grinds of pepper; mash until creamy.  Set aside.

Heat 1 T oil in a large skillet.  Add beef and 1/2 tsp salt; cook, undisturbed, until browned on bottom, about 4 minutes.  Break up pieces with a wooden spoon and cook until browned all over, about 5 minutes.  Transfer to a bowl.  Wipe's out skillet.

Cook mushrooms, thyme and 1/2 tsp salt in 1 T oil over medium-high, stirring occasionally, until golden, about 5 minutes.  Transfer to book with beef.  Heat remaining 1 T oil over medium.  Add onion, celery, and garlic; cook, stirring occasionally until golden, about 5 minutes.  Return beef mixture to skillet; stir in tomato paste and cook 1 minute more.  Add stock and remaining 1/2 tsp salt; simmer until slightly thickened, 7 to 8 minutes.

Stir in carrots, peas, and Worcestershire; cook until peas are crisp-tender, 3 to 4 minutes.  Transfer mixture to a 9-by-13 inch baking dish.  Spoon, then gently spread mashed potatoes on top.

Set baking dish on a rimmed baking sheet.  Bake until filling is bubbling and top is golden in spots, 18-22 minutes.  Let stand 15 minutes before serving.