Preheat oven to 375°. Reserve 1/2 cup raspberries. In a small saucepan, combine water, brown sugar, vinegar and remaining raspberries; bring to a boil. Reduce heat; simmer, covered, until sugar is dissolved and raspberries have broken down, 4-6 minutes, stirring occasionally. Press raspberry mixture through a fine-mesh strainer into a bowl; discard seeds. Stir in 1/2 teaspoon salt and 1/8 teaspoon pepper; return to pan. Bring to a boil. Reduce heat; simmer, uncovered, until liquid is reduced to 1/2 cup, 2-3 minutes., In a small bowl, stir butter until smooth. Stir in remaining salt and pepper. Spread butter mixture over fillets., In a large cast-iron or other ovenproof skillet, heat oil over medium heat. Add fillets; cook 1-2 minutes on each side. Bake until fish just begins to flake easily with a fork, 6-8 minutes. Serve with sauce. Top with reserved raspberries and, if desired, mint.