Place the eggs in a saucepan and cover them with cold water by about 1 inch.

Bring the water to a boil, then cook the eggs for 9 minutes for firm hard-boiled yolks.

Drain the eggs and cool them under cold running water until easy to handle.

Peel the cooled eggs and chop them into small pieces.

Halve the avocado, remove the pit, and scoop the flesh into a mixing bowl.

Mash the avocado with a fork until mostly smooth, leaving a few small chunks for texture.

Finely chop 1 to 2 tablespoons of onion.

Add the chopped eggs and 2 heaping tablespoons of tuna in oil to the mashed avocado.

Add 2 tablespoons mayo and 1 teaspoon Dijon mustard to the egg mixture.

Stir in the chopped onion, 1 teaspoon chili crisp, and black pepper until evenly combined.

Spread the butter over one side of each of the 4 bread slices.

Heat a skillet over medium heat and place the bread slices in the skillet, buttered-side down.

Place 1 slice of cheese on each bread slice and cook until the undersides are golden and the cheese melts.

Transfer the cheesy toast to a cutting board and divide the egg salad evenly over 2 slices.

Close the sandwiches with the remaining cheese toast, slice each sandwich in half, and serve immediately.