Pat the pork loin dry with paper towels, then season well on all sides with kosher salt and ground black pepper.
Add two tablespoons of oil to a large heavy pan (preferably cast iron) and warm over high heat. Add the pork loin to the pan and sear a few minutes on each side, turning as needed, until the pork develops a nice golden color on all sides and ends.
Remove the pork loin from the pan and let it rest/cool while you prepare the herb butter.
Preheat the oven to 375 degrees Fahrenheit.
Put the softened butter in a small bowl, and use a fork to mash it together with the garlic, basil, salt, pepper, and lemon zest.
When the pork has cooled enough so that you can comfortably touch it, place it on a roasting rack in a large roasting pan. Use your hands to rub the butter mixture all over the top of the pork loin.
Next, starting at one end of the pork loin and working your way to the other, lay the slices of speck across the pork, tucking any loose ends underneath the sides of the loin. Let the speck slices overlap slightly as you lay them across the pork. They will shrink when they crisp up, so you want the slices overlapping to avoid any weird gaps in the final product.
Place the pork in the oven and roast for about an hour and a half, or until the center of the loin reaches 140 degrees Fahrenheit (use a ThermaPen or other instant read thermometer to check).
When the loin is done cooking, remove it from the oven and let it rest for 15 minutes before slicing and serving. The rest will allow the roast finish cooking and to allow the juices to redistribute.
When the loin is done resting, slice it carefully into crosswise slices, taking care to serve the crispy speck pieces with each slice.