Preheat the oven to 425F.
Slit the pork tenderloin vertically. Open it up, spread on the work surface, cover with cling film and pound with a kitchen mallet to a thickness of a quarter inch. Discard the cling film.
Sprinkle salt and pepper over the entire surface of the meat. Scatter the tarragon on the meat.
Arrange the sausages and cheese in two lines, side by side. The two lines should ideally be as long as the length of the pounded meat.
Roll up the meat to envelope the filling.
Butter a large square of foil. Place the rolled meat at the center and gather the foil around it, crimping the sides.
Bake at 425F for 30 minutes.
Open up the foil. Spread butter on the meat. Keep the meat loosely covered. Let the meat rest for 10 minutes. Cut into chunky slices. Serve.