Preheat your oven to 200°C (390°F).
Cook the Beef:
In a large pan over medium-high heat, brown the beef, breaking it into chunks as it cooks. Season with salt and pepper. Once browned, use a slotted spoon to transfer the beef to a plate. Discard the excess fat from the pan.
Sauté the Vegetables:
Reduce heat to medium. Add the butter, oil, leek, onion, carrots, and celery. Season with salt and pepper, and cook until softened—about 10 minutes. Add the garlic and cook for 1 more minute. Stir in the tomato paste and cook for another 2 minutes.
Wine time:
Pour in the wine, scraping up any browned bits from the bottom of the pan. Let it simmer until reduced by about half.
Make the Filling:
Return the beef to the pan. Add the beef stock, Worcestershire sauce, and herb bundle. Stir well and let it simmer for 12–15 minutes, or until the liquid has thickened significantly. Taste and adjust seasoning as needed. Remove the herb bundle and stir in the peas.
Mash the potatoes:
You know what to do!
Assemble the Casserole:
Transfer the meat mixture to a 9×13 inch (or similar-sized) baking dish. Spread the mashed potatoes evenly over the top and fluff the surface with a fork. Sprinkle panko breadcrumbs over the potatoes.
Bake:
Bake for 25–30 minutes, or until the top is golden.
Rest and Serve:
Let the dish rest for 5 minutes before serving.