This is one of those winning combinations of ingredients that makes it difficult to fail. Make an appetizer or a meal of it... you'll be happy either way.
1 frozen puff pastry sheet, approximately 11 x 17"
1 medium sweet onion, thinly sliced
1/2 green apple, peeled and cut into thin slices
3 to 4 thin slices prosciutto
1 cup grated gruyere cheese
1 egg yolk mixed with 1 tsp water
freshly ground black pepper
chopped fresh rosemary and thyme to garnish, about 2 tsp
Heat oven to 400f.
Lay the puff pastry out onto parchment paper lined baking sheet.
Let thaw a bit and then score the puff pastry around the perimeter, about 1/2" from the edge.
Brush the ed with egg yolk mixture and sprinkle with the turbinado sugar, kosher salt and pepper.
Bake the puff pastry for 12-14 minutes, until light golden brown.
Meanwhile caramelize the onions by simply slowly sautéing them in olive oil until they are golden brown and tender. Then add the apple and cook for another minute. Let cool.
To assemble, spread the onion and apple mixture over the puff pastry. You may not need all of it, use as much as you like.
Take the prosciutto slices and roughly tear them apart and spread them over top of the onions.
Sprinkle the gruyere cheese over top.
Turn the oven up to 425f.
Bake again for another 7-10 minutes until cheese is melted.
Sprinkle with rosemary and thyme and cut to serve.