Sweet Potato Garlic Fries

Name: Sweet Potato Garlic Fries
Ingredients: 3 tablespoons olive oil; 2 red onions, thinly sliced; 1 tablespoon balsamic vinegar; 1/4 teaspoon kosher salt; 1/4 teaspoon freshly ground black pepper; 2 1/2 pounds sweet potatoes or garnet yams (about 3 medium), scrubbed, unpeeled and cut into 1/4-inch-by-4-inch fries, soaked in ice water; Canola oil or vegetable oil; 2 1/2 cups all-purpose flour; 1/2 teaspoon baking powder; 1/2 teaspoon baking soda; 1/2 teaspoon kosher salt; 1/4 cup (1/2 stick) unsalted butter, at room temperature; 1 tablespoon chopped fresh thyme; 3 cloves garlic, minced; 8 ounces shredded fontina, for serving