Cut a lengthwise slit down the center of the tenderloin to within 1/2- inch of bottom (careful not to cut all the way through).
Open tenderloin so it lies flat. On each half, make another, lengthwise slit down the center to within 1/2-inch of bottom; cover with plastic wrap. Flatten to 1/4-in. thickness with a mallet.
Remove plastic wrap; spread 1/2 of the garlic/Dijon mixture on the inside of the pork. Layer the cheese, spinach, sun dried tomatoes and prosciutto on top, press down gently.
Roll up pork jelly-roll style, starting with a long side. Tie the roast at 1-1/2-inch to 2-inch intervals with kitchen string.
Season lightly with salt, and remaining garlic/Dijon mixture. Place on a rack in a shallow baking pan.
Bake, uncovered, at 425° for 25-30 minutes or until a meat thermometer reads 160°. Place under the broiler to brown the top for 5 minutes, then transfer to a serving platter or cutting board.
Let stand 10 minutes before removing string and slicing. Slice in 8 pieces.