Mix butter, rosemary, and mustard in small dish. Season with salt and pepper. Preheat broiler. Brush tuna on both sides with oil and sprinkle with salt and pepper. Spread some rosemary butter on 1 side of each baguette slice. Broil tuna to desired doneness, about 4 minutes per side for medium. Broil baguette slices until beginning to brown, about 1 minute per side. Top each tuna fillet with dollop of rosemary butter. Serve with baguette slices.