Start by preheating your oven to 200°C (392°F). Grab a large skillet and heat up the extra virgin olive oil over medium heat. Once it’s hot, toss in the diced onions. Sauté until they turn translucent, about 5 minutes. This will set a flavorful foundation for your Shepherd's Pie.

Next, add the diced carrot and sliced celery to the skillet. Stir them around for about 3-4 minutes until they become slightly tender. Now, sprinkle in the fresh rosemary, stirring it in to release its aromatic oils.

Add the lean ground lamb to the skillet. Use a spatula to break it up. Cook until browned, generally around 8-10 minutes. Incorporating the vegetables with the meat at this stage adds layers of flavor.

Now, pour in the beef stock and canned diced tomatoes. Give everything a good stir, making sure to combine all ingredients. Let this simmer for 10 minutes so that the flavors meld beautifully. You can also throw in the garlic powder and smoked paprika here for that extra umami kick.

While the filling simmers, put the peeled and cubed potatoes in a pot of salted water. Bring it to a boil and cook until they're fork-tender, around 15 minutes. Drain the potatoes and return them to the pot. Here’s where you’ll add the skim milk and reduced-fat margarine. Mash until smooth and fluffy, then mix in the grated Parmesan cheese.

In a baking dish, spoon the meat and vegetable filling in an even layer. Carefully dollop the mashed potatoes on top. Spread it evenly using a spatula. If you're feeling adventurous, create some swirls and peaks with a fork—this helps it look inviting and will give it a nice texture when you bake.

Transfer your baking dish to the preheated oven. Bake for about 30-35 minutes until the top is golden brown. The aroma wafting through your kitchen will be nothing short of incredible.

Let the Shepherd's Pie cool for about 5-10 minutes before serving. This waiting time allows the layers to settle and makes it easier to cut. Enjoy your creation with a side of green salad or steamed veggies!