Peel your hard-boiled eggs and chop them into small chunks—not too fine or they'll get mushy. Place all the chopped eggs in a medium bowl and set aside. Don't panic if a few shells stick; just rinse them under cold water and they'll come right off.

Add your mayonnaise, Dijon mustard, and lemon juice to the bowl with the eggs. Mix everything together gently with a fork, stirring just until combined. You want some texture here, so don't over-mix or you'll end up with mush.

Fold in the chopped celery, red onion, and fresh parsley with a few gentle strokes of your fork. Taste the egg salad sandwich filling and add a pinch more salt and pepper if you want. I always taste and adjust because everyone's salt preference is different.

Spread butter on one side of each bread slice—this is the side that'll touch the skillet. This step makes the crust golden and crispy, which honestly transforms this sandwich completely. Trust me, don't skip it.

Heat a skillet over medium heat and place one buttered slice in there, butter-side down. You'll hear it sizzle right away, which means it's at the perfect temperature. Let it toast for about 60 seconds until it gets golden brown.

Remove the toasted bread from the skillet and top it with cheese, then pile on your egg salad sandwich mixture. Top with the second bread slice, buttered-side up, and carefully transfer the whole thing back to the warm skillet. Toast the outside for another 60 seconds until it's golden and the cheese starts melting.

Flip gently and toast the other side for about 60 seconds until it's equally golden. The cheese should be all melty by now and everything should hold together nicely. Slice diagonally (because it just tastes better that way) and serve immediately.