Preheat a skillet over medium heat.
In a small bowl, mix together the mayonnaise and Dijon mustard. Spread this mixture evenly on one side of each slice of sourdough bread.
On the unspread side of one slice, layer the turkey slices, crumbled bacon, and cheddar cheese. Season with salt and pepper to taste.
Place the second slice of bread on top, spread side facing out.
Melt the butter in the preheated skillet. Carefully place the sandwich in the skillet and cook for about 3-4 minutes, or until the bread is golden brown.
Flip the sandwich and cook for an additional 3-4 minutes until the other side is golden and the cheese is melted.
Remove from the skillet, let it cool for a minute, then slice in half and serve.