Put apple, lemon juice and sugar into a small pot, cover and cook over medium heat, stirring occasionally, until apples are very soft and liquid is absorbed, 8 to 10 minutes. Purée in a food processor along with mustard and salt; set aside to let cool. Spread mustard on half of the rye squares and arrange a few leaves of arugula on top. Top each with a wedge of Brie, a dollop of apple mustard and the remaining rye squares and serve. The apple mustard can be made a few days ahead and stored in the fridge.
Go to full version of