Preheat the oven to 400˚F. Combine the panko bread crumbs and Parmesan cheese in a small bowl. Set aside.
Heat a large, deep skillet over medium-high heat, and brown the ground beef, using a wooden spoon to break it up into evenly-sized pieces. Cook, stirring frequently, until completely brown, with no pinkish pieces. Sprinkle with salt and pepper. With a slotted spoon, transfer the ground beef to a plate, pressing the beef against the pan to drain the fat away. Discard the remaining fat in the pan.
Melt the butter in the same pan, lowering the heat to medium. Add the olive oil, leek, onion, carrots, and celery. Sprinkle with salt. Cook, stirring occasionally, for 8-10 minutes, until softened. Stir in the garlic and cook for 1 more minute. Stir in the tomato paste, and cook for 2 more minutes, stirring frequently. Pour in the wine, and use a wooden spoon to scrape up any browned bits on the bottom of the pan. Simmer for 3-4 minutes, until the wine has reduced by half. Return the ground beef back into the skillet, and fold to combine. Add the beef stock and Worcestershire sauce and stir well. Tie the thyme and bay leaf together with kitchen twine, and add to the skillet. Simmer 12-15 minutes, until the liquid thickens. Remove the thyme and bay leaf. Taste and adjust the seasoning if necessary, and fold in the peas.
Transfer the meat mixture to a 9x13-inch casserole. Dollop the mashed potatoes on top, and spread them out in an even layer. Sprinkle the top with the reserved panko and cheese mixture, then bake for 25-30 minutes, until the top is golden brown. If you'd like an extra crispy crust, set under the broiler for 1-2 minutes (but keep a watchful eye on it!). Allow the shepherd's pie to sit at room temperature for 10 minutes before serving.