Put oven rack in middle position and preheat oven to 400°F.
Cut scallions diagonally into 2-inch pieces.
Whisk together remaining ingredients except pork in a large bowl.
Add scallions and pork, turning pork to coat generously with sauce.
Arrange pork in 1 layer in a 15- by 10-inch shallow baking pan (1 inch deep). Spoon remaining sauce with scallions over pork and roast until just cooked through, 15 to 20 minutes.
Turn on broiler and broil pork 5 to 6 inches from heat until top is slightly caramelized, 2 to 5 minutes.