Heat a deep-sided sauté pan over medium heat. Once hot, add the ground lamb and cook, breaking it apart and allowing it to brown and caramelize.
1.65 pound ground lamb
Meanwhile, cut the carrots and celery into a fine five-millimeter dice. Finely dice the shallots, crush the garlic, and pick and finely chop the thyme and rosemary.
2 carrot, 2 celery stalk, 2 shallot, 2 garlic clove, 2 sprig fresh thyme, 2 sprig fresh rosemary
Set a strainer over a bowl and drain the browned lamb, reserving some of the rendered fat.
Return the pan to the heat and add a small amount of the reserved lamb fat. Add the shallots, carrots, celery, garlic, thyme, and rosemary. Cook until the vegetables begin to soften.
Stir in the tomato paste, flour, and a pinch of salt. Cook for about 30 seconds.
2 teaspoon tomato paste, 1 tablespoon all-purpose flour, kosher salt
Return the lamb to the pan. Pour in the red wine and simmer until reduced by about half.
1 cup dry red wine
Add the chicken broth. Reduce the heat to low and simmer for about 30 minutes, or until the liquid has reduced by roughly half and thickened into a rich sauce.
3 cup good-quality chicken broth
Stir in the Worcestershire sauce and frozen peas. Taste and adjust the seasoning as needed.
2 tablespoon Worcestershire sauce, ⅓ cup frozen peas
Transfer the filling to an oven-safe baking dish. Let it cool slightly until a skin begins to form over the surface. This helps prevent the mashed potatoes from sinking into the filling.