►Melt butter in a large skillet over medium-low heat. Add onions and salt and cook, stirring occasionally, until deep golden and collapsed, about 30 minutes. Deglaze onions with balsamic vinegar, then remove from heat and cool.
►Meanwhile, add eggs to a saucepot and cover with water by 1 inch. Bring to a full, rolling boil over high heat, uncovered. Once water is boiling, cover pot, remove from heat, and let stand 6
minutes. Strain eggs from water and let cool slightly before peeling.
►Slice eggs in half and remove yolks. Place yolks in a food processor with onions, cream cheese, red wine vinegar, and smoked paprika. Process until mixture is fluffy and fully incorporated.
►Pipe filling into egg whites, then garnish with chopped chives. Serve immediately.