In a medium bowl, mash the eggs with a potato masher until desired consistency.
Add the mayonnaise, scallion, and mustard to taste. Stir gently to combine. Season to taste with salt and pepper.
Lay out four slices of bread. Divide the egg salad mixture among the slices. Divide the bacon and sprinkle atop the egg salad. Layer tomatoes over the bacon. Place a small handful of greens on each.
Top with the remaining slices of bread. Slice in half and serve.
Notes
Note: If packing for take-away, consider packing the egg salad separately in small dishes and assembling right before eating.