In a large skillet, heat oil over medium-hot heat. Add onion and garlic and cook until onion is tender but not brown. Remove skillet from heat. Stir in spinach, bacon and Parmesan cheese; set aside.
Trim excess fat from pork. To pinwheel the pork loin, start cutting lengthwise 1/2 inch under the fat cap on the roast. As you cut the roast, gently rotate the roast at the same time. This will allow the roast to "unroll" as you cut it. When the roast is completely cut, it should be a rectangular piece of meat between 1/2 and 1-inch thick.
Spread spinach mixture over cut side. Roll up the loin tightly from a long side to resemble the initial roast. Tie securely with 100 percent cotton kitchen string. Place the roast on a rack in a roasting pan; insert a meat thermometer into thickest part.
Roast in 325 degree F oven for 1-1/2 to 1-3/4 hours or until meat thermometer registers 150 degrees F. Transfer roast to a serving platter. Cover loosely with foil and let stand for 15 minutes before carving. (The temperature of the meat after standing should be 160 degrees F.) Remove string; cut into 1-inch-thick slices.