Special equipment: butcher's twine
For the filling: Heat the olive oil in a large saute pan over medium-high heat until hot. Add the garlic and onion and saute until translucent. Add the sausage and bay leaves and cook, stirring and breaking up the meat with the back of your spoon, until the meat is browned off and the onion is tender, about 8 minutes. Add the white wine and rosemary and cook until reduced by half. Transfer the mixture to a bowl to cool.
Preheat the oven to 425 degrees F.
For the pork: Meanwhile, while the filling is cooling, butterfly your pork loin by cutting through the roast lengthwise almost all the way through but leaving 2 inches so it can open like a book. Open up the pork and lay it flat, then pound the pork with a meat mallet until it's roughly 1 1/2 inches thick. (You can also ask your butcher to do this for you.) Season the pork on both sides with salt and pepper.
Combine the olive oil, thyme, fennel and garlic together in a small bowl. Rub half of the mixture on one side of the pork.
Pat the cooled sausage filling down across the pork in an even layer and sprinkle with the chopped parsley. Roll it up tightly and tie at 1 1/2-inch intervals using butcher's twine. Rub the outside of the pork all over with the rest of the olive oil-fennel rub.
Place the roast fat-side up on a rimmed sheet tray and add the beer to the bottom of the pan. Roast the pork for 30 minutes, then lower the heat to 350 degrees F. Continue roasting until golden and crisp on the outside and an instant-read thermometer registers 145 degrees F, 40 minutes more. Allow the roast to rest for 20 minutes before removing the twine and slicing.