Combine 1/4 cup water, sugar, and fish sauce in a large nonstick skillet; bring to a boil, stirring to dissolve the sugar. Add lemongrass, garlic, and pepper. Cook 1 1/2 minutes or until slightly reduced. Add 1/4 cup water, onions, and fish, and cook over medium-high heat 7 minutes or until fish flakes easily when tested with a fork, turning once. Sprinkle with cilantro.