Make marinade for short ribs. Peel and cut Asian pear into quarters. Place one quarter in a blender and set aside remaining pear. Add onion, olive oil, balsamic vinegar, honey, garlic, ginger, and black pepper and puree until smooth. Pour marinade into ziploc bag along with short ribs. Marinate for at least 8 hours and up to 24 hours, turning the bag once halfway through.
Prepare shiitake rub. Grind dried shiitake mushrooms and black peppercorns in spice grinder until finely ground. Mix with salt, sugar, and red chili flakes until well combined.
Preheat oven to 425 degrees F. Line baking sheet with parchment paper or aluminum foil and place wire rack on top.
Remove short ribs from marinade, scraping off excess. Pat dry with paper towels. Sprinkle generous amount of shiitake rub on both sides and let sit at room temperature for 30 minutes.
Place short ribs on prepared baking sheet with wire rack. Bake for 8 minutes, flip short ribs over and bake another 8 minutes or until caramelized.
Meanwhile, make salad. Thinly slice remaining pear and combine with sliced scallions. Whisk together red chili flakes with sesame oil, rice vinegar, sesame seeds, and sugar and pour over pear and scallions. Toss gently to combine.
Serve shiitake crusted short ribs with spicy pear and scallion salad.