To make the rub, working in batches, finely grind the porcini in a spice grinder and transfer the ground porcini to a medium container or bowl. Finely grind the peppercorns and red pepper flakes and add them to the container bowl with the porcini. Add the salt and sugar and stir to combine.
To marinate the ribs, separate the layers of the onion and slice 1⁄4-in. thick lengthwise. Using a sharp knife, score the meat of each short rib on both sides in a 1⁄4-in. diamond, crosshatch pattern, making sure not to cut all the way through the meat; combine ribs with the onion in a large baking dish.
In a blender, combine the balsamic vinegar, oil, honey, and kiwi and purée. Pour the marinade into the dish or bags, dividing it evenly. Massage the marinade into the meat with your hands. Refrigerate the ribs for at least 2 hours and no more than 4 to marinate.
To make the salsa verde, combine the oregano, mint, capers, lemon juice, kosher salt, and red pepper flakes in a medium bowl. Using a fine Microplane grate the garlic into the bowl and stir to combine. Gradually add oil, stirring constantly.
Remove the short ribs from the refrigerator and uncover. Discard the marinade and onion. Scrape off the excess marinade with your hands and lay the ribs in a single layer on a large baking sheet or cutting board. Pat both sides of the ribs dry with paper towels. Sprinkle the porcini rub on both sides of the ribs and massage to thoroughly coat the ribs. Allow ribs to come to room temperature for about 30 minutes.
Prepare a hot fire in a wood-burning or charcoal grill or preheat a gas grill for high heat. When grill is hot, arrange short ribs about 1 in. apart. Grill the short ribs until they have black grill marks on both sides, 3 to 4 minutes per side. Remove the ribs from the grill and place them on a large dry baking sheet.
To serve, spoon salsa verde over the ribs. In a bowl, toss scallions with 2 tsp. oil and 1⁄2 tsp. kosher salt and sprinkle over the ribs.