Add peeled russet potato chunks to salted water at 212°F and cook for 15 to 18 minutes. Press one chunk against the pot side with a spoon, and when it breaks without a hard center, drain it right away. Mash with unsalted butter and warm whole milk for 2 minutes using firm downward presses, then fold in fine sea salt with a rubber spatula. The mash should look fluffy with a few soft ridges. Go past 18 minutes and the potatoes turn waterlogged and gluey.

Cook the ground lamb in a cast iron skillet over medium high heat, about 7 on a 10-dial, for 6 to 8 minutes. Break it into uneven crumbles with the edge of a spoon and push the browned bits through the onion and carrot as they soften. The lamb should turn from pink to deep brown while the yellow onion looks glossy and the carrot edges soften. You'll smell the meat get nutty right near minute 7. Stop too early and the filling tastes flat because the lamb hasn't browned.

Stir in garlic clove, tomato paste, all purpose flour, beef stock, Worcestershire sauce and fresh thyme leaves over medium heat, about 5 on a 10-dial, for 4 to 5 minutes. Drag the spoon across the skillet base in slow circles so the tomato paste darkens and the flour disappears into the stock. The gravy should coat the lamb and leave a short trail when pulled through the center. You'll see slow bubbles instead of thin splashes. Rush this window and the flour tastes dusty, but cook it too long and the bottom starts to catch. (I used to pour in extra stock here - it made the pie slump when sliced.)

Lower the heat to 300°F and fold in the frozen green peas for 1 to 2 minutes. Use broad scooping motions from the skillet edge toward the center so the peas stay mostly whole and scattered through the filling. Transfer the filling into a ceramic baking dish and spread it into a slightly uneven layer about 1 inch deep. The peas should stay bright green against the darker gravy. Overcook them and they turn dull and mushy before the pie even bakes.

Spoon the mashed russet potato over the lamb filling while the oven is at 375°F, then spread it for 3 to 4 minutes in short strokes from the center outward. Leave ridges and small gaps at the edges so steam can escape. Scatter shredded sharp cheddar cheese unevenly across the top and rough up the surface with the back of the spoon. The topping should look bumpy and cloudlike, not smooth. Press too hard and the lamb gravy pushes through the potato.

Bake the Shepherd’s Pie at 375°F for 22 to 25 minutes. Place it in the center of the oven and turn the dish once at minute 15 so the potato peaks brown more evenly without looking perfect. The top should have golden patches and the edges should bubble with a little lamb gravy. You'll hear a faint sizzle when it comes out. Pull it before 22 minutes and the center stays loose, but go past 25 minutes and the edges can dry out.

Rest the baked Shepherd’s Pie at 200°F carryover warmth for 10 to 12 minutes before serving. Cut down through the potato with one clean scoop motion, then lift one portion so the filling stays tucked under the topping. The slice should show soft mashed russet potato, browned cheese spots and a thick lamb layer with carrots and peas. Skip the rest and the gravy runs across the plate instead of holding in the serving.