These make-ahead sheetpan sandwiches are a fast and easy way to feed a crowd. Set them aside and pop them in the oven whenever you’re ready to get the party started. Level them up with a dense Belgian or artisan loaf for a wonderful crust and crunch or try with a rye or sourdough bread for great flavour.
In a large skillet over medium heat, melt butter. Add onion, thyme and salt; cook, stirring often for about 20 minutes or until onion is very soft and golden brown. Set aside.
Preheat oven to 450 F (232 C). Line a baking sheet with parchment paper; set aside.
Spread butter on 1 side of each slice of bread. Press some of the Appenzeller cheese to 4 of the buttered sides and place cheese side down on prepared pan. Spread pesto on those 4 slices of bread. Top with onions and brie cheese. Top each with remaining bread slices, butter side up. Sprinkle with remaining Appenzeller and press sandwiches down slightly.
Bake for 8 minutes and remove from oven. Using a flat spatula, flip sandwiches carefully and press down again slightly. Return to oven for about 5 minutes or until golden brown and cheese is melted. Let cool slightly before serving.