1. Preheat oven to 400F. Spray 12-inch skillet (nonstick recommended) with canola oil; heat over medium-high heat 1-3 minutes or until shimmering. Add onions and sugar to skillet; cook, uncovered, 8-10 minutes or until onions are light brown, stirring often. Add vinegar; cook 2 minutes, stirring occasionally. Remove skillet from heat.
2. Meanwhile, in a 2-qt mixing bowl, combine Gruyere cheese and garlic; set aside. Sprinkle a medium sheet pan (about 9×14) with bread crumbs. Unfold pastry sheet onion lightly floured pastry mat. Using a rolling pin, roll out pastry into a 14×9 inch rectangle. Place pastry onion prepared sheet pan, pressing up sides.
3. Sprinkle pastry with Parmesan cheese. Arrange half of the potatoes over pastry, slightly overlapping. Sprinkle potatoes with half EACH of the rosemary, salt, pepper, and Gruyere cheese mixture. Top with remaining potatoes, rosemary, salt and pepper. Sprikley with onions and remaining Gruyere cheese mixture. Bake 35-40 minutes or until edges of pastry are golden brown. Remove sheet pan from oven to a cooling rack. Cut tart into 12 pieces (for full recipe); sprinkle with parsley.