On a cutting board, spread one side of each of your slices of bread with mayonnaise.
Place two of them mayo-side-down in a non-stick pan with the heat off.
Top with cheddar slices, overlapping to cover entire slice of bread, followed by pickled onions, then scallion microgreens, then some grated cheddar or gruyere. Sprinkle with a pinch of ground black pepper.
Close sandwiches with second slice of bread mayo-side-up.
Turn heat to medium and cook until golden brown and crisp, about 5-7 minutes, being careful not to burn.
Using a spatula, carefully flip the sandwiches. Season with salt, reduce heat and cook until cheese is melted and bread is evenly browned, about 4-5 minutes more.
Slice and serve immediately.