Begin by peeling and quartering the russet potatoes, then place them in a large pot. Cover them with cold water and add a generous pinch of salt. Bring the water to a boil, then reduce the heat and simmer for 22 to 25 minutes, or until the potatoes are very tender. While the potatoes cook, preheat your oven to 375°F (190°C), preparing it for the final bake.

Heat 3 tablespoons of olive oil in a large braiser or Dutch oven over medium heat. Add the diced white onion, chopped carrots, and sliced celery, along with a pinch of salt and black pepper. Sauté for 3 to 5 minutes until the vegetables begin to soften. Next, add the lean ground beef to the pan, breaking it up and cooking it for 4 to 5 minutes until it's no longer pink, ensuring you drain any excess fat. I always make sure to brown my meat thoroughly at this stage for maximum flavor.

To the browned beef and vegetables, stir in the minced garlic and Italian seasoning, cooking for about 30 seconds until fragrant. Then, add the Worcestershire sauce and tomato paste, stirring well to combine. Sprinkle the flour over everything and cook for another minute, then gradually pour in the beef broth, stirring continuously to prevent lumps. Simmer the filling uncovered for 5 to 10 minutes, or until the sauce thickens and the carrots are tender. This slow simmer really allows the flavors to meld together beautifully!

Once the beef filling has thickened and the carrots are tender, turn off the heat and stir in the frozen peas, fresh parsley, and fresh thyme. Keep this warm while you prepare the mashed potatoes. Drain the cooked potatoes from Step 1 thoroughly. Return them to the pot and add the unsalted butter and room temperature milk. Mash until smooth, then season generously with salt and black pepper to taste. For truly creamy mashed potatoes, I like to use a potato ricer if I have one handy.

To the mashed potatoes, add the freshly grated sharp white cheddar cheese and chopped chives, mixing just until combined. Transfer the warm beef filling into a baking dish. Spoon the cheesy mashed potato topping evenly over the beef filling, spreading it to the edges. Bake in the preheated 375°F (190°C) oven for 25 minutes, then broil for 2 to 4 minutes until the potato topping is lightly browned and bubbly. Allow the cottage pie to rest for 10 minutes before serving; this helps it set and makes for cleaner slices.