Preheat the broiler to high.
For the mustard: Place the arugula, salt and garlic in a food processor. Pulse until finely chopped. Add the mustard and maple syrup and puree until smooth. Transfer to a container and refrigerate until ready to use. (The mustard can be made up to 2 days in advance.)
For the crostini: Slice the baguette on a steep bias into 1/4-inch slices; it should yield about 18 to 20 slices. Brush each slice with some of the olive oil and place on a baking sheet. Sprinkle with the salt and broil until golden brown and crunchy, 5 to 6 minutes. Set aside to cool.
To assemble, spread 1/ 2 teaspoon of the arugula mustard onto each crostini. Top with a ribbon of cucumber and a piece of prosciutto, folded into a ribbon-like shape. Serve.