Brown the Lamb: Heat olive oil in a large Dutch oven or heavy-bottomed skillet over a medium-high heat. Brown the ground lamb in small batches, shaped into loose balls, to ensure good caramelization. Remove the browned meat and set aside, draining off excess fat if needed.
Build the Flavor Base: In the same pan, add the onions, carrots, and celery. Cook until softened and golden brown (about 8-10 minutes). Add the garlic, thyme, rosemary, salt, and pepper, and cook for another 1 to 2 minutes until fragrant.
Simmer the Filling: Stir in the tomato paste and cook for two minutes. Pour in the red wine to deglaze the pan, scraping up any browned bits, and reduce slightly. Return the lamb to the pot with the stock, and bring to a simmer. Reduce heat to low, partially cover, and let simmer for 1 hour, stirring occasionally.
Finish the Filling: Remove from the heat and stir in the frozen peas and Worcestershire sauce. Season with salt and pepper to taste.