To make phyllo tartlet shells: Preheat oven to 325 degrees F. Lightly coat 2 mini-muffin pans with nonstick cooking spray. In a small bowl, whisk together egg white, oil and salt.
Lay a sheet of phyllo on a work surface and with a pastry brush, lightly coat it with the egg-white mixture. Lay a second sheet smoothly on top, taking care to line up the edges before setting the sheet down. (Once you set down the sheet, it cannot be moved.) Brush with the egg-white mixture and repeat with 1 more sheet. Lay a fourth sheet on top but do not brush it.
With a knife, cut the dough into 4 strips lenghtwise and 5 strips crosswise, making 24 squares. Press squares into muffin cups and bake for 8 to 12 minutes, or until golden brown and crisp. Transfer the tartlets to a rack and let cool. Repeat the procedure with the remaining 4 sheets of phyllo and egg-white mixture. (The baked tartlet shells may be stored in a closed container at room temperature for 1 week or in the freezer for up to 2 months.)
To make smoked trout filling: In a food processor, combine cream cheese and smoked trout; process until fairly smooth. Add scallions and horseradish and pulse until just combined. (Alternatively, finely mince the smoked trout with a knife and combine with the cream cheese, scallion and horseradish in a small bowl.) (The smoked filling may be made ahead and refrigerated for up to 2 days.) Shortly before serving, spoon or pipe about 1 heaping tsp. of filling into each tartlet shell and garnish with shredded cucumber.
50 calories per piece: 3 g protein, 2 g fat, 5 g carbohydrate; 94 mg sodium; 5 mg cholesterol.
The rich, creamy filling contrasts with the pleasant crunch of the tartlet shell.