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 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 tomato paste, and cook for 2 more minutes, stirring frequently. Pour in wine, and use a wooden spoon to scrape up any browned bits on the bottom of the pan. Simmer for 3-4 minutes, until liquid has reduced by half. Return the ground beef to skillet, and fold to combine. Add beef stock and Worcestershire sauce, stir well. Tie thyme and bay leaf together with kitchen twine, and add to the skillet. Simmer 12-15 minutes, until liquid thickens. Remove thyme and bay leaf. Taste and adjust the seasoning if necessary, and fold in the peas.
Transfer 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 reserved panko and cheese, then bake for 25-30 minutes, until top is golden brown. For an extra crispy crust, set under the broiler for 1-2 minutes. Allow the shepherd's pie to sit at room temperature for 10 minutes before serving.