Spray 2 large rimmed baking sheets with nonstick spray. Divide onion wedges between prepared baking sheets. Brush onions on both sides with 2 tablespoons oil; sprinkle with 4 teaspoons thyme, salt, and pepper. Arrange onions cut side down and roast until golden brown on bottom, about 10 minutes. Turn onions over. Roast until golden brown and tender, about 10 minutes longer. Transfer to another large bowl.