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1
Preheat the oven to 400 degrees F.
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2
Bring water, vinegar and brown sugar to a boil.
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3
Add the onions and remove from the heat.
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4
Cover, and set aside until the onions come to room temperature.
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5
Trim the crusts off the bread and cut the bread into thirds, for 3 rectangles per piece.
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6
Lightly brush both sides of the bread pieces with olive oil and lay out on a baking sheet.
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7
Toast in the oven until just crisp and lightly brown, about 5 minutes.
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8
Cool.
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9
To assemble: Quarter, core, and very thinly slice the apple, (a mandolin is terrific for this).
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10
Very thinly slice the cheese into triangles or shards that will fit nicely on the bread.
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11
Place a dab or smear of mustard on the bread.
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12
Place a piece of apple, cheese and 1 or 2 sprigs of watercress on each piece of toasted bread.
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13
Wrap a small piece of prosciutto around each stack to make a neat package.
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14
Top with a little bit of the pickled onions, a drizzle of olive oil and freshly cracked black pepper.
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15
Serve at room temperature.
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16
Do ahead tips: Toast the bread ahead, and thinly slice the apple, placing them in lemon water to prevent them from turning brown.
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17
If you need to assemble the sandwiches ahead, don't add the watercress, and cover them with a very light moistened paper towel.
