Adjust toaster oven cooking rack to the lowest position. Place an ungreased baking sheet inside the toaster oven and preheat to 450°F on the “Bake” setting.
Butter two bread slices and place buttered-side-down on a cutting board. Top each with a slice of cheese and layer with the walnuts, arugula, and apple slices. Tear the last slice of cheese into pieces and distribute evenly across both sandwiches. Spread remaining bread with condiment of choice, place on top and butter outsides.
Using oven mitts, carefully remove the preheated pan. Place sandwiches on the pan and return to the toaster oven.
Cook sandwiches about 4 to 5 minutes. Carefully remove the pan, flip sandwiches with a spatula and return to the toaster oven. Cook until the bread is golden brown and the cheese is melted, about 3 to 4 minutes.