Rinse and dry one large ripe avocado. Cut it in half, remove the seed, and score the flesh into cubes. Scoop out the avocado into a bowl, drizzle with fresh lemon juice, and sprinkle some salt to taste. Mash until creamy and smooth.

In a nonstick skillet, melt 1 tablespoon of unsalted butter over medium heat. Toast 2 slices of large white bread until light golden brown, about 3-4 minutes. Repeat with remaining slices.

On the toasted side of each slice of bread, layer two slices of provolone cheese. Spread half of the creamy avocado mixture on one slice, top with 2 more slices of cheese, and cap with another slice of bread, toasted side down.

Melt an additional tablespoon of butter in the skillet over medium-low heat. Place both assembled sandwiches in the skillet and cook until deep golden brown, about 5 minutes per side.

Remove the sandwiches from the skillet. Cut each sandwich in half and serve immediately for the best experience.