1. For relish, in a small saucepan over medium heat, bring onion and water to a boil. Cook and stir for 1 minute; drain.
2. Transfer to a bowl; add roasted peppers, vinegar, garlic, oregano, salt, pepper and cumin.
3. Let stand at room temperature for 1 hour. Discard garlic. (Relish can be made ahead and stored in the refrigerator for up to 1 week.)
4. In a small bowl, mash the minced garlic, cumin and salt. Rub over tenderloin; place in a shallow baking pan.
5. Bake, uncovered, at 425° for 25-30 minutes or until a meat thermometer reads 160°.
6. Let stand for 10 minutes; thinly slice pork.
7. Heat oil in a large skillet over medium heat. Top four slices of bread with pork, desired amount of relish and two slices of cheese; top with remaining bread. Cook sandwiches for 2-4 minutes on each side or until golden brown.