In a large skillet over medium heat, cook the bacon until it is crispy and golden. Transfer it to a paper towel lined plate to drain. Leave about a tablespoon of the rendered fat in the skillet for extra flavor, or wipe it clean if you prefer.

Spread a thin layer of softened butter on one side of each slice of sourdough bread. Place the bread, butter side down, in the same skillet over medium heat. Toast for about 1 to 2 minutes until the bottom is light golden brown. Remove and set aside, toasted side up.

On the untoasted side of four bread slices, spread a little mayonnaise or Dijon mustard if desired. Layer one slice of cheddar cheese, two slices of turkey, two strips of bacon, and a few slices of avocado, lettuce, or tomato if using. Top with another slice of cheddar cheese. Place the remaining bread slices on top, toasted side facing outward.

Return the sandwiches to the skillet over medium low heat. Cook for 3 to 4 minutes per side, pressing gently with a spatula, until the bread is deep golden brown and crispy, and the cheese is fully melted. The smell of butter and toasted sourdough will fill your kitchen.

Remove the sandwiches from the skillet and let them rest for a minute before slicing diagonally. This helps the cheese set slightly and prevents it from oozing out too quickly. Serve warm with your favorite sides.