Cook lamb over med-high heat, breaking up, until there is no pink and fat has rendered, about 5 minutes. Drain in a fine-mesh strainer set over a bowl to reserve drippings.
Return 2 T. of drippings to pot. Heat over med-low heat until shimmering. Add onion, carrots and 1/2 t. salt. Cook until softened and lightly browned, 7-10 minutes.
Stir in garlic and tomato paste; cook until fragrance about 30 seconds. Stir in flour and cook until incorporated, about 60 seconds. Whisk in broth and wine, scraping up browned bits, until sauce is smooth.
Stir in reserved lamb, worcestershire, thyme, and rosemary. Coook until it begins to thicken, about 2 minutes. Off the heat, stir in peas. Season to taste with salt and pepper. Transfer mixture to a 2 to 2.5 quart baking dish. Set aside.
Meanwhile, cover potatoes by one inch of water. Add 1 T. salt. Bring to a boil, then reduce to a simmer. Cook until taters are tender and a fork easily slips into the center, about 10-12 minutes. Drain. Return to pan over low heat. Mash them to a smooth consistence. Stir in butter and cream. Season with salt and pepper to taste.
Dallop potatoes over filling and smooth into an even layer, making sure they attach to the edges of the casserole dish. You shouldn't be able to see any filling. Place on a foil-lined baking sheet. Bake at 450 for about 30 minutes, until edges bubble and top is lightly browned.
Cool for 10 minutes before serving.