Preheat the oven to 200°C | 390°F. Place the turkey breast in a large roasting pan lined with parchment paper, skin side up. Pierce through the skin and flesh with a fork, all over the breast. Set aside.: The kitchen fills with a faint warm scent as the oven reaches temperature, and that initial heat is what jumpstarts the browning process. Piercing the skin helps the rub penetrate, allowing oil and seasonings to mingle with the meat so flavors infuse more deeply as it roasts. A common mistake is skipping the skin piercing, which can prevent the seasoning from reaching the meat and produce a less flavored center.
In a small bowl, combine 4 tablespoons olive oil, garlic, parsley, thyme, paprika, salt, and pepper. Rub HALF of the mixture evenly all over the skin of the turkey breast. Loosen the skin and rub the seasoning underneath the skin and onto the meat. Pour the extra 2 tablespoons oil underneath the turkey breast onto the parchment paper. Turn the breast over, skin-side down.: As you mix the herbs and oil, their aroma becomes vivid, releasing herbal, garlicky notes that promise depth. Rubbing half on the surface and slipping seasoning under the skin ensures both a crisp, seasoned exterior and moist, flavored meat beneath. Pouring oil beneath the breast helps the underside brown and creates flavorful drippings. Avoid piling the seasoning only on top, since that concentrates flavor only at the surface.
Arrange the sweet potatoes and potatoes around the turkey, and rub them with remaining seasoning. Season with extra salt, if desired.: When the potatoes meet the hot pan, their outer edges will begin to desiccate slightly, which leads to the satisfying caramelization you want. Coating them with the remaining rub ties their flavor to the turkey, so every bite echoes the same herb notes. Make sure pieces are similar size so they cook evenly, otherwise some may become overdone while others remain firm.
Cover pan completely with aluminium foil. Roast for 2 hours.: Sealing the pan locks in steam and concentrates juices, producing tender meat and helping the potatoes soften uniformly. During this phase the turkey steams gently, and you may hear a soft hiss as moisture circulates under the foil. A common pitfall is leaving the foil loose, which lets heat escape and dries the meat, so create a tight seal for best results.
Remove from oven, carefully uncover the turkey, and turn in the pan to be skin-side up. Change oven setting to grill or broil, and continue to roast the turkey for a further 30 minutes – 45 minutes, or until the skin is crispy and the turkey is completely cooked through.: Taking the foil off reveals a steamed, pale surface that will transform under high heat into crisp, browned skin. The broiler intensifies surface caramelization, producing audible crackles and an irresistible toasted scent. Keep a close eye to avoid burning, and check internal temperature so the turkey reaches a safe doneness without overcooking.
When the turkey is done, remove from the oven, cover the pan with the same foil, and allow to rest at room temperature for 15 minutes. Slice and serve warm with the gravy.: Resting lets juices redistribute, resulting in slices that are moist rather than weepy. You will notice the aroma mellow and the meat firm slightly, which makes carving easier. A frequent mistake is slicing immediately, which causes precious juices to run out and leaves the meat drier on the plate.
Pour out all the pan juices into a medium-sized pan and bring to a simmer. Mix in the broth and continue to simmer over low heat.: Simmering concentrates flavors and allows fat to separate, giving you control over the gravy’s body. As it simmers you will smell the roasted potatoes and herbs becoming more pronounced, an encouraging sign that the sauce is building depth. Don’t rush the simmer or the flavors will not meld fully.
Pour in the corn starch mixture while whisking continuously, until a nice and thick gravy begins to form.: Adding the slurry while whisking prevents lumps and creates a glossy finish that clings to the turkey slices. You should see the sauce transform from thin and slightly oily to a velvety coating that screens spoonfuls across the plate. If it thickens too much, thin with a splash of broth, and be careful not to boil vigorously after adding starch, which can break down the thickener.