Preheat grill to medium-high heat.
Using aluminum foil, create an oblong cooking tray (approximately 13 inches by 8 inches by 2 inches) to be placed directly on grill. Place salmon fillet in center of tray. Season first with garlic salt, sprinkle with brown sugar, and then cover with pieces of butter. Top with sliced red onions. Pour beer of choice into tray to just below the highest point of the fillet. Cover tray with aluminum foil to envelope fish completely. Place tray on grill, cover with lid, and grill for approximately 8 minutes or until just cooked through.