Heat a grill or large skillet over medium-low heat.
Butter one side of each slice of bread. Spread the mustard over the opposite side on 4 slices of the bread. Top each of the 4 slices with the Cheddar, crisped bacon and apple slices. Add the remaining slices of bread on top with the buttered side facing out.
Place the assembled sandwiches in the skillet. Cook until the cheese begins to melt and the bread is golden brown. Flip and cook the other side until golden brown as well.
Remove from the skillet, slice and serve.