In a medium bowl, stir together the mustard and 1 tablespoon water until smooth. Let stand for 15 minutes. Stir in the salt until well combined.
Meanwhile, in a medium saucepan, boil the cherries, red wine vinegar, port, and sugar over high heat, stirring occasionally, until syrupy, about 10 minutes. Transfer to a blender and puree until smooth.
Return the mixture to the saucepan and bring to a boil over high heat. Boil, stirring occasionally, until the consistency of ketchup, about 5 minutes. Stir the cherry mixture into the mustard mixture, a little at a time, until completely incorporated. This mustard will keep in the refrigerator for up to 3 days.
Heat your grill to medium-high. Use a lightly oiled kitchen towel to carefully grease the grill grate.
In a small bowl, stir together the cumin, sherry vinegar, and honey. Reserve 1 tablespoon in another bowl and use the rest to brush all over the pork. Let the pork stand for 5 minutes while the grill heats.
Grill the pork, turning every 45 seconds to cook evenly, until the center is still a little pink, about 8 minutes. Remove from the grill, brush with the reserved honey mixture, and let rest for 10 minutes. Serve with the cherry mustard.