In a medium bowl combine the marinade and brown sugar. Stir to combine. Reserve 1/2 cup of the marinade for brushing on the pineapple and vegetables. Pour the remaining marinade in a zip top plastic bag. Add the steaks and press out the air before sealing the bag. Tip the bag over on itself to completely cover the steaks. Lay them flat in the refrigerator for at least 1 hour and up to 4, flipping the bag every 30-60 minutes.
Meanwhile, alternate threading the pineapple, bell pepper and red onion onto metal or pre-soaked wooden skewers. Place the skewers on a baking sheet along with the scallions and reserved marinade. Refrigerate until ready to grill.
Remove the steaks from the marinade and place them on a baking sheet. Pour the steak marinade into a bowl and reserve for basting. Dry the steaks off on both sides using a paper towel. (You want the steaks to char when they first hit the grill and if they're too wet, they'll steam). Let the steaks rest at room temperature while you grill the vegetables.
Heat the grill to medium high heat (about 450 degrees). Place the vegetable skewers on the grill and brush with marinade. Flip the skewers after 3-4 minutes and brush on more marinade. Cook the vegetables until crisp tender and slightly charred. Transfer to the baking sheet and tent with foil.
Heat the grill to a high heat (about 525-550 degrees) Place the steaks on grill and lower the temperature to a medium high heat, about 450 (watch for flare-ups). Cook steaks for 7-9 minutes for medium rare - medium, turning them about halfway through the cooking. Grill the green onions for about 5 minutes total. Baste both the steak and the green onions with reserved marinade.
Transfer the steak and scallions to a cutting board and let the meat rest for 5 minutes before cutting.
Slice the steak into 1" slices and arrange on a platter. Pour any collected juices back onto the meat. Slide the vegetables from the skewers and arrange on the platter along with the grilled onions. Serve.