Heat oven to 425°F. Spray large cookie sheet with cooking spray.
In 10-inch skillet, melt butter over medium heat. Add onion; cook about 15 minutes, stirring occasionally, until onion is golden brown. Stir in brown sugar and thyme. Cook 1 to 2 minutes, stirring occasionally, until sugar is dissolved. Remove from heat.
Unroll pizza dough; place on cookie sheet. Starting at center, roll or press dough into 15x10-inch rectangle. Sprinkle cheese down center third of dough. Spoon onion mixture evenly over cheese.
With scissors or sharp knife, make cuts 1 inch apart on both long sides of dough to within 1/2 inch of filling. Alternately cross strips diagonally over filling; turn ends under and press to seal. Brush egg white over top. Sprinkle with poppy seed.
Bake 11 to 15 minutes or until crust is golden brown. Let stand 5 minutes before serving. Remove from cookie sheet; cut crosswise into slices.