Preheat oven to 300 degrees F. Peel and coarsely shred potatoes; immediately rinse with cold water in a colander. Drain well, pressing lightly, then pat dry with paper towels; place in a large bowl. Quarter the onion slices. Stir onion, oil, thyme, salt, and pepper into potatoes.
Lightly coat an unheated very large nonstick skillet or griddle with nonstick cooking spray. Preheat skillet or griddle over medium heat. For each cake, scoop a slightly rounded measuring tablespoon of the potato mixture onto skillet or griddle. Press down potato mixture with a spatula to flatten evenly. Cook for 5 minutes. Using a wide spatula, carefully turn potato cakes (be sure not to turn cakes too soon or they will not hold together). Cook for 3 to 5 minutes more or until golden brown. Place cooked potato cakes on a baking sheet. Keep warm, uncovered, in oven while cooking remaining potato cakes. Repeat with remaining potato mixture, stirring mixture frequently. Makes 8 servings (16 potato cakes).