Pickle the shallot
In a large bowl, toss the shallot with the vinegar and a pinch of salt. Set aside for 10 minutes.
Toast the bread
Melt the butter (or oil) in a large, nonstick skillet over medium-high. Add the Kaiser rolls, cut-side down, and cook until golden, 2 to 3 minutes. Remove from the skillet and set aside.
Cook the JUST Egg
Add JUST Egg to the skillet and cook, stirring occasionally with a rubber spatula, until curds start to form, about 2 minutes. Stir in the cheese and continue to cook until the cheese has melted and the eggs are thick, 1 to 2 minutes more. Season with salt and pepper. Place half on the bottom cut-side of one of the Kaiser rolls, and the rest on the bottom cut-side of the other roll.
If usingJUST Egg Folded, heat according to package directions.
Build the sandwich
Add the spinach and olive oil to the bowl with the shallot and season with salt and pepper. Place on top of the eggs, along with half an avocado. Add cucumber slices on the other half of the roll. Serve immediately.