Bring a large pot of water seasoned with a generous amount of salt to boil. Cook pasta according to package instructions. Reserve ½ cup pasta water for later.
In a large saucepan, melt duck fat over medium heat. Add garlic, rosemary, and red chilli flakes. Cook for roughly a minute or two until fragrant.
Add in shallots. Season with salt and pepper. Saute for about 5 minutes.
Stir in the reserved pasta water. Bring to a boil and simmer for 5 minutes.
Toss in cooked Tagliatelle pasta. Add in the lemon zest and Mozzarella cheese. Mix to combine until creamy.
Garnish with fresh parsley leaves. Top with reheated leftover roast duck slices and more grated cheese if desired. Serve immediately.
