Sprinkle both sides of the pork medallions with a little salt and pepper.
Heat a lidded sauté pan over medium-high heat. Add 1 tablespoon neutral vegetable oil to the pan.
Sear the medallions in the oil until nicely browned on each side, about 3 minutes per side.
Add chicken or vegetable stock, jam, mustard, ginger and vinegar to the pan. Bring to a simmer, cover, and simmer over low heat until the internal temperature of the pork is 155-157 degrees F, about 15-20 minutes, depending on your stove.
Remove the pork to a platter and cover with foil.
Return the pan to the heat, and turn the heat up to medium high. Bring to a boil and reduce the sauce until thick and syrupy. Taste, and adjust seasonings, if necessary.
Remove from the heat and swirl in the butter, just until melted. This will enrich the sauce a bit and give it a really nice sheen.