Hard cook eggs: Place eggs in a pot and fill with room temperature water until all the eggs are completely submerged. Bring to a boil then turn down to a low boil for approximately 12 minutes.
Once the eggs have cooled down, peel and chop into four pieces each.
Place kale and eggs in a large bowl.
Add oil to a pan with the onions and heat, stirring, for 2 minutes.
Add mushrooms and cook, stirring, until softened, about 2 minutes more.
Toss in string beans (and corn if using) and heat for about a minute.
Remove from the heat and stir in tomato, vinegar, mustard, pepper and salt.
Pour the mushroom mixture over the kale and eggs.