Set oven rack on the upper level; preheat to 400 degrees F. Lightly coat a baking sheet with nonstick cooking spray or line with parchment paper In a small bowl, whisk together the egg white and 2 Tbsp. olive oil.
Lay a sheet of phyllo on the prepared baking sheet, and with a pastry brush, lightly coat the surface with the egg-white mixture. Sprinkle with 1 tsp. breadcrumbs. Repeat this step, layering 4 more sheets of phyllo on top and brush with egg-white mixture. To form an edge to the tart, carefully roll over the edges toward the center, using the blade of a knife to help you get started.
With a rubber spatula, spread mustard over the surface of the dough and sprinkle with cheese. (The tart can be prepared ahead to this point.
Wrap and freeze for up to 2 months. Do not thaw before continuing.) Arrange tomato slices on top in 5 rows of 8 slices each. Bake for 15 to 20 minutes, or until the pastry is golden brown.
Let cool in the pan for 5 minutes. In a small bowl, combine the remaining 1 Tbsp. olive oil, parsley, garlic and thyme. With your fingers or a fork, dab some of the herb mixture onto each tomato slice. Slide the tart onto a serving platter or, if you wish to serve bite-sized appetizers, slide it onto a cutting board and with a sharp knife or pizza cutter, cut the tart into squares between the tomato slices. Serve warm or at room temperature.
32 calories per piece: 1 g protein, 1 g fat, 4 g carbohydrate; 40 mg sodium; 1 mg cholesterol.
"The tangy mustard is a pleasing accent to the sweet tomatoes in this easy-to-prepare appetizer."