Preheat oven to 400°F. Heat oil in a large skillet over medium-low. Add salt, pepper, and onions; sauté 15 minutes or until onions are lightly browned. Add sugar, vinegar, and chopped thyme; cook 5 minutes or until liquid evaporates. Remove pan from heat; cool slightly. Roll pie dough into a 12-inch circle on a lightly floured surface. Place thyme sprigs in bottom of a 9-inch cake pan. Spread onion mixture in bottom of pan in a single layer. Drape dough over pan, folding edges underneath. Prick surface liberally with a fork. Bake at 400°F for 20 minutes. Reduce oven temperature to 350°F (leave pan in oven); bake at 350°F for 15 minutes or until dough is browned and crisp. Remove pan from oven; cool 15 minutes. Carefully invert tart onto a serving plate; cut into 8 wedges.