Cook the potatoes in boiling water in a large pot until tender, about 30 minutes. While waiting for the potatoes to cook, you can work on the next steps.
Heat olive oil in a large pan over medium-high heat. Add beef, rosemary, thyme, salt, and pepper. Cook until the beef is no longer pink, breaking up the meat with a wooden spoon and stirring frequently, about 5 minutes.
Add diced onions, celery, and carrots to the pan. Cook until the vegetables soften, about 5 minutes, stirring occasionally.
Add beef stock, tomato paste, and Worcestershire sauce to the pan, stirring well as it simmers. Reduce heat to medium-low and let it continue to simmer until the sauce is thicker, about 15 minutes.
When the potatoes have finished cooking, drain well. Add butter and parmesan cheese to the pot with the drained potatoes. Mash until smooth.
Add the meat mixture to a 2 quart baking dish, flattening it along the bottom. Spoon the potatoes over the meat, using a fork to rough up the potato surface. Drizzle with olive oil.
Bake at 400 F until the peaks of the potatoes are crispy, about 30 minutes. Garnish with chives and freshly ground black pepper.