Raisin bread lends sweetness to a chopped liver sandwich.
Melt butter in a medium saute pan over medium heat. Season liver with salt and pepper, and dust with flour, shaking off excess. Cook liver until golden brown and no longer pink in center, about 4 minutes per side. Transfer to a plate, and let cool; reserve pan.
Cook onion in reserved pan until tender, 4 to 5 minutes. Transfer onion to a medium bowl, and let cool completely. Chop liver, and add to onion. Stir in mayonnaise, and egg whites, and season with salt and pepper. Refrigerate 30 minutes (or up to 1 day).
Spread mayonnaise on 4 slices of bread. Divide chopped liver and arugula or watercress among bread slices. Sandwich with remaining bread.