Add bread flour, spelt flour, sourdough starter, sugar, and 1 cup plus 2 tablespoons of the water to the bowl of a stand mixer. Mix with a paddle attachment on low until combined, about 1 minute. Cover with a kitchen towel and let sit at room temperature for 45 minutes.

Switch to a dough hook and uncover bowl. With the mixer on low, add salt and remaining 2 tablespoons water (about 30 ml). Add oil in a slow stream and mix until everything is evenly incorporated, about 1 minute. Let rest at room temperature, covered, until dough increases slightly in volume, 3 to 4 hours, stretching and folding dough back over itself in bowl every 30 minutes. Cover with plastic wrap and refrigerate for 24 hours.

Grease a large rimmed baking sheet with oil. Remove dough from refrigerator and transfer to greased baking sheet, and stretch the dough to the edges of the pan. Cover dough loosely with plastic wrap, and let proof at room temperature until it is nearly doubled in size and springs back slowly when pressed with your finger, 1 to 3 hours.

Strain tomatoes in a fine-mesh strainer over a large bowl until liquid has slowed to a drip, about 15 minutes. Discard juices. Transfer tomatoes to a medium saucepan and stir in salt, olive oil, and herbs. Simmer over medium-low, stirring often, until reduced slightly, about 30 minutes. Remove from heat and cover to keep warm; you will have about 1 ¾ cups sauce.

While preparing and cooking the sauce, preheat oven to 500°F; let preheat for at least 30 minutes before baking. Once dough is proofed, generously cover with 2 tablespoons olive oil and dimple the surface with your fingertips to ensure an even structure.

Bake in preheated oven until the top and bottom are golden-brown and dough begins to pull away from edges of the pan, 15 to 20 minutes. Remove from oven and let cool on wire rack for 5 minutes before slicing and serving.

Serve Sfincione with pizza sauce on the side for dipping or topping.