Heat 2 tablespoons of the olive oil in a large skillet over high heat. Add the onion and cook for 1 minute. Add the spinach, pushing it down into the skillet, and ¼ teaspoon each of the salt and pepper. Cover and cook over medium heat for about 1 ½ minutes, until the spinach is wilted. Remove the lid and cook, uncovered, until the liquid from the spinach has evaporated. Transfer to a plate and let cool.

Trim the tenderloin of any fat and silverskin. Butterfly the tenderloin for stuffing, pound it to about 12 inches long by 7 inches wide.

Preheat the oven to 350 degrees. Arrange half the spinach mixture down the center of the butterflied tenderloin and top with the cheese. Add the rest of the spinach, fold in the sides, and roll the tenderloin back and forth to evenly distribute and encase the filling. Wrap 2 strips of aluminum foil, each 1 to 2 inches wide, around the tenderloin to secure the stuffing inside.

Heat the remaining 2 tablespoons oil in a large ovenproof nonstick skillet. Sprinkle the outside of the tenderloin with ¼ teaspoon each of the salt and pepper. Place the tenderloin carefully in the skillet and brown it, turning occasionally, for about 5 minutes. Carefully remove the foil strips from the tenderloin and bake in the oven for 10 minutes, when it will be slightly pink in the center. Transfer the tenderloin to a plate, cover, and keep warm in the oven while you prepare the tomatoes (the pork will continue to cook as it sits).

Add the tomatoes and the remaining ¼ teaspoon each salt and pepper to the skillet in which you browned the tenderloin and sauté over high heat for 1 ½ to 2 minutes, until just softened. Divide among four warm plates.

Slice the tenderloin crosswise into 8 medallions and arrange 2 slices in the middle of the tomatoes on each plate. Serve.