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