Preheat oven to 350 degrees F. Coat a large, heavy baking sheet with cooking spray.
In a small bowl, pour boiling water over tomatoes. Let stand until tomatoes are slightly soft (2 - 3 minutes). Drain and pat dry. Chop finely.
In a large bowl, sift together flours, sugar, salt, baking powder and cayenne. Add the egg and egg white and thyme. Stir vigorously to combine (dough will be very stiff). Add walnuts and chopped tomatoes and work them into the dough.
Turn out dough on a lightly floured surface and knead a few times. Pat and roll into a 10-inch square about 1/4 inch thick. With a sharp knife, cut dough crosswise into quarters and place the pieces at least 1 inch apart on the prepared baking sheet.
Bake 10 minutes. Turn over and bake until dry and lightly browned (8 - 10 minutes longer). Transfer to a wire rack to cool completely.
To serve, break into smaller, irregular pieces.