For the Pork:

1. Preheat the oven to 350°F. Place a rack in the center of the oven.
2. Pat the pork dry with paper towels. Mix together the honey, garlic, thyme, and rosemary. Rub the mixture evenly over all sides of the pork. Season evenly with the salt and pepper.
3. Heat the olive oil in a large skillet over medium-high heat. Sear the meat on all sides to get a slightly browned crust without burning the garlic, about 1-2 minutes per side.
4. Remove the loin from the skillet and place in a Dutch oven. Pour the Guinness around the loin. Cover with the lid and transfer to the oven. Braise for 45 minutes to 1 hour, or until the thickest part of the meat reaches an internal temperature of 145°F. Let rest for 10 minutes before slicing and serving.

For the Guinness Gravy (Optional):

1. As the pork is resting, make the gravy by first removing 1/2 cup of braising liquid from the Dutch oven.
2. Melt the butter in a medium saucepan over medium heat. Add the diced onions and 1/4 teaspoon salt, and saute until slightly golden, about 3-5 minutes.
3. Sprinkle the flour over the onions and stir well to evenly coat. Cook over medium heat, stirring constantly, for about 2 minutes.
4. Pour in the stock and braising liquid and whisk until thoroughly combined. Cook for another 2 minutes, stirring constantly.
5. Reduce the heat to low and stir in the cream. Cook for another 2-5 minutes to slightly thicken the mixture. Season with the remaining salt and black pepper and serve with the pork.