Brie, Apple, and Caramelized Onion Panini

I had this sandwich in Boston several years ago and I've remembered it fondly ever since.  Then, it was so unusual to me to have fruit with cheese, and on a hot sandwich, too!  Now, it's so common that every sandwich shop around here has a version.  But I still think it's wonderful.

1 large yellow onion, sliced thinly
1 teaspoon sugar
2 tablespoons butter
pinch of salt
1 firm apple (Granny Smith, Fuji), sliced thinly
4 slices thick bread
4-6 ounces brie






Melt the butter over medium low heat in a dutch oven and add onions.  Sprinkle with the sugar and salt, and cook onions, stirring frequently, until dark brown.  10-20 minutes.
Assemble sandwiches with caramelized onions, brie, and apple slices.  Use panini press to cook sandwiches, or just place sandwiches on a buttered skillet and weigh them down with something heavy like a saucepan.   Flip each sandwich and repeat on the other side.  Makes 2 sandwiches.