Pluck 1 Tbsp. leaves from a sprig of rosemary. Reserve the rest. Roughly chop the rosemary; makes it easier to form the paste
Using a mortar and pestle (or knife and cutting board), combine the rosemary leaves, 4 cloves of smashed garlic, 1/2 tsp. fennel seeds and a pinch of salt and freshly ground pepper. Pound or chop until a coarse paste forms.
Season the pork liberally with salt, then coat with the garlic-rosemary mixture. Wrap in foil and refrigerate the pork for at least 2 hours and up to 8.
When ready to cook, bring the pork to room temperature. Heat 3 Tbsp. olive oil in a large, heavy-bottomed skillet. Add the pork and sear for 4 to 5 minutes until browned.
Flip the pork over and add all the mushrooms. Salt and pepper the mushrooms, add a few more rosemary sprigs, and sear the pork for another 3 to 4 minutes. If it looks at all dry, add another splash of olive oil.
Add 2 tsp. red wine vinegar and a splash of white wine and stir well to combine and incorporate bits from the bottom of the pan.
Place in the heated 425 degree oven and finish roasting for about 10 minutes until pork reaches 125 to 130 degrees.
Let rest in loosely tented foil. Slice and serve topping with mushrooms and any pan drippings over the top.