2 tbsp apple cider vinegar (TRY: Bragg Organic Apple Cider Vinegar)
1/2 tsp ground ginger
Prepare jam: In a large saucepan, combine tomatoes, honey and salt. Bring to a simmer on medium-high. Reduce heat to medium-low and continue to simmer until most of juice evaporates. Reduce heat to low, cover saucepan and cook for 25 minutes, stirring occasionally, until tomatoes soften. Add vinegar and ginger and simmer, uncovered, for 10 to 15 minutes, or until slightly thick. Cool at room temperature for about 30 minutes before using. (Jam may be made up to 3 days ahead; cover and refrigerate.)
Preheat oven to 450°F. Coat a baking sheet with cooking spray. Place pork on baking sheet and mist again with cooking spray. Season all sides of pork with chile powder, oregano and pepper. Roast for 20 to 25 minutes, until pork is no longer pink in center or an inserted meat thermometer reaches 145°F. (Pork tenderloin may be roasted up to 2 days ahead; wrap tightly and refrigerate.) Let pork rest for 10 minutes, then thinly slice.
Top each of 4 bread slices with 1 oz cheese, quarter of pork, 2 tbsp jam, quarter of nectarines and quarter of spinach. Place remaining bread slices over top.
Preheat a panini press, griddle or 2 heavy skillets to medium to medium-high and coat with cooking spray. Place sandwiches on cooking surface (if using skillets, place 2 sandwiches in each) and toast until golden brown, adjusting heat as needed. If using a griddle or skillets, mist top sides of sandwiches with cooking spray, then flip and repeat on opposite side.