Make the Crust: Grease a 9x13-inch baking dish; set aside.
In a large bowl, whisk together the flour, baking powder, salt and baking soda until well combined. Add the butter and, using a pastry blender, cut in butter until it's the size of small peas. Pour in cold buttermilk and stir mixture with a wooden spoon just until a dough forms (do not overmix).
Transfer dough to a lightly floured surface; gently pat into a rough ball. Use a sharp knife or bench scraper to cut dough in half. Place one half on top of other half, and very gently press halves together, shaping into a rough ball again. Repeat 3 times.
Gently shape dough into a rough rectangle about 11x14 inches. Gently transfer the dough to the prepared baking dish and press into the bottom, corners and up the sides. Refrigerate while you prepare the sauce.
Preheat oven to 425 degrees F.
Make the Sauce: Heat the butter in a medium saucepan over medium heat until melted. Add onion, oregano, and salt; cook, stirring occasionally, until liquid has evaporated and onion is golden brown, about 5 minutes. Add garlic and cook until fragrant, about 30 seconds. Stir in tomatoes and sugar, increase heat to high, and bring to a simmer. Lower heat to medium-low and simmer until reduced to about 2½ cups, 25 to 30 minutes. Off the heat, stir in the basil and olive oil, then season with salt and pepper.
Bake the Pizza: Remove the dough-lined pan from the refrigerator and sprinkle with the shredded mozzarella cheese. Layer on the pepperoni and sausage, if using. Spread about half to three-quarters of the sauce over the toppings (you can freeze the leftover sauce) and sprinkle with Parmesan cheese. Bake until the crust is golden brown, about 30 minutes. Remove pizza from oven and let rest 10 minutes before slicing and serving.