Preheat the oven to 450.
Slice the bread about ¾ inch thick into 12 equal slices. Butter one side of the bread slices and place 6, butter side down, on a rimmed sheet pan.
In a small bowl, mix the two cheeses with the thyme, kosher salt and several grinds of black pepper. Spread about ⅔ of the cheese evenly over the 6 bread slices, then top with the candied nuts. Next layer on a few slices of apple, and finally add the remaining cheese.
On the remaining 6 slices of bread, spread a layer of mustard onto the NON buttered side, and place mustard side DOWN onto the sandwiches. Place in the oven and bake for 3-4 minutes.
Remove from the oven and using a spatula, carefully flip the sandwiches over, pressing down slightly with the spatula. Return to the oven for another few minutes. If the bread isn’t crisping up, turn on the broiler to brown the sandwiches. Be careful as the bread can burn easily, especially if it is too close to the broiler. Watch it the whole time, and remove from the oven once it is the desired color.
Cut in half and serve immediately.