If not using lingonberry sauce from a jar, place the foraged lingonberries in a small saucepan with the sugar and water. Bring to a boil over medium heat and simmer for about 10 minutes or until thickened into a sauce.
Trim the pork, removing the silver skin. (If using one large tenderloin, rather than two small ones, cut in half.)
For the marinade, mix together the lingonberry sauce, dijon mustard, and white wine. Place the pork in a shallow dish and pour over the marinade. Cover with plastic and refrigerate for at least an hour.
Once the pork has marinated, heat the oven to 350°F. Place an oven safe skillet over medium to medium-high heat and add the olive oil. When the pan is good and hot, sear the pork well on both sides, setting aside and reserving the marinade.
After the pork has been seared on both sides, pour the reserved marinade into the pan. Sprinkle the chopped rosemary over the top and place the skillet in the oven.
Roast the pork for 20 to 30 minutes, depending on its thickness, or until the internal temperature reaches 145° to 150° F. When the pork is done, transfer it from the skillet to a carving surface and cover with foil. Allow the pork to rest while finishing the gravy.
Place the skillet with its remaining contents over medium heat and gently stir in the salt and ruby port. Simmer and reduce the mixture until thickened, about 10 minutes. Taste for seasoning and add more salt, if necessary.
Slice the pork on the diagonal and serve with the lingonberry gravy spooned over the top. Garnish with a bit more chopped fresh rosemary, if desired.