In a blender, combine scallions, vinegar, soy sauce, ginger, garlic, jalapeno, 2 tablespoons oil, thyme, allspice, nutmeg, and 1/4 teaspoon ground pepper. Puree until smooth. Place marinade in a resealable plastic bag. Add pork, turning to coat. Refrigerate for at least 1 hour and up to 24 hours.
Heat grill to high. Oil grates. Lift pork from marinade. Season with salt. Grill 8 to 10 minutes per side or until an instant-read thermometer registers 145 degrees. Let cool 5 minutes.
Meanwhile, in a large saucepan, heat remaining oil over medium. Add chard ribs and cook, stirring occasionally until crisp-tender, about 10 minutes. Add leaves, salt, and pepper; cook, stirring occasionally, until tender.
Pork imparts B vitamins, while leafy Swiss chard boosts vitamins A and C.