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 teaspoon breadcrumbs. Repeat this step, layering 4 more sheets of phyllo on top. Lay the sixth sheet of phyllo on top and brush with the 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. Spread mustard over the surface of the dough with a rubber spatula and sprinkle with cheese. Arrange tomato slices on top in 5 rows of 8 slices each.