French Onion Tart

Name: French Onion Tart
Ingredients: 1 tablespoon extra virgin olive oil; 2 pounds yellow onions, peeled & sliced thin; 2 sprigs fresh thyme, leaves removed & stems discarded; Sea salt and freshly ground pepper, to taste; 1 large egg, lightly beaten; 1 teaspoon sherry vinegar; 2/3 cups (2 ounces) Swiss cheese, shredded; 3/4 cups (110 grams) whole wheat pastry flour; 1/4 teaspoon (2 grams) fine sea salt; 1/4 teaspoon paprika; 1 cup (3 ounces) Swiss cheese, shredded; 4 tablespoons (2 ounces) very cold butter, shredded using a box grater; 1 to 2 tablespoons cold seltzer