Rach's tip: I make this shepherd's pie with lamb, but beef is just as delicious!
Position a rack in the center of the oven; preheat to 400°.
Peel the potatoes and cut each into about twelve 3/4-inch pieces. Place in a medium saucepan and cover with cold water. Bring to a boil over high heat.
Arrange the bacon on a slotted broiler pan or a wire rack set inside a baking sheet lined with foil. Bake until very crispy, 15 to 18 minutes. Preheat the broiler.
When the water comes to a boil, salt it liberally. Cook the potatoes until tender, 8 to 10 minutes. Drain and return the potatoes to the hot pot. Add the milk, chives, and sour cream; season with salt and finely ground white or black pepper. Mash the potatoes. Chop the bacon and mix half into the potatoes. Cover the pot to keep warm.
While the potatoes are boiling and the bacon is baking, heat the olive oil, two turns of the pan, over medium-high in a large skillet. Add the lamb and cook, stirring occasionally and breaking up the meat with a spoon, until browned, about 5 minutes. Stir in the onion, carrot, and celery; season with salt and coarsely ground black pepper. Cover and cook, stirring occasionally, until the vegetables soften, 3 to 4 minutes. Add the stock, garlic, thyme, Worcestershire, and tomato paste. Simmer, stirring occasionally, until the flavors meld, about 2 minutes. Stir in the peas and cook, stirring often, until heated through, about 2 minutes.
Transfer the filling to a 3-quart casserole dish. Top with the mashed potatoes, then both cheeses. Broil the casserole until bubbly and browned in spots, 2 to 3 minutes. Top with the parsley, paprika, and the remaining bacon. Drizzle with the hot honey.