Melt 2 tbsp butter in a large skillet over medium-high heat. Add onion and stir until golden, 5-7 minutes; transfer to plate. Add 1 tbsp butter and livers to pan; cook until just pink inside. Add to onion. Deglaze pan with cognac and remaining 2 tbsp butter. Using food processor, finely chop livers, onion, eggs and pan sauce; season. Spread on toast or crackers.