In a bowl mix the onion and vinegar. Add a little more vinegar if needed to make sure the onions are somewhat submerged. Then set aside. Drain the vinegar before you are ready to use the onions.
Add the eggs to a saucepan and cover with water. Bring water to a boil, boil the eggs for 8 minutes. Drain the water and fill the pot with cool tap water. Let the eggs sit until you are ready to peel.
Lightly toast the bread.
Peel the eggs and add them to a bowl. Add mayonnaise, paprika, dill, scallions, and mustard. Then mash with a fork, season with salt and pepper to taste. Adjust any of the ingredients as needed to taste.
Layer the bread with spring mix, tomato seasoned with salt and pepper, egg salad and top with onions. Lightly season with cracked pepper.
Serve with pickles.
Notes
Pro Tip: Depending upon your appetite make 1/2 sandwich with the soup