1. Process Gruyère, Brie, and wine in food processor until smooth paste is formed, 20 to 30 seconds. Add chives and pulse to combine, 3 to 5 pulses. Combine butter and mustard in small bowl.

2. Working on parchment paper-lined counter, spread mustard butter evenly over 1 side of slices of bread. Flip 4 slices of bread over and spread cheese mixture evenly over slices. Top with remaining 4 slices of bread, buttered sides up.

3. Preheat 12-inch nonstick skillet over medium heat for 2 minutes. (Droplets of water should just sizzle when flicked onto pan.) Place 2 sandwiches in skillet; reduce heat to medium-low; and cook until both sides are crispy and golden brown, 6 to 9 minutes per side, moving sandwiches to ensure even browning. Remove sandwiches from skillet and let stand for 2 minutes before serving. Repeat with remaining 2 sandwiches.

Serves 4