1) To make the crust: Combine the dry ingredients and the oils and butter, mixing till crumbs form. Then add the water, and mix and knead — by hand, stand mixer, or bread machine set on the dough cycle — to make an elastic, fairly stiff dough.
2) Place in a large, greased bowl; cover, and let rise at room temperature for 1 hour. For best flavor, after its initial 1-hour rise, refrigerate the dough for several hours, or for up to 24 hours. You can use the crust after its first 1-hour rise, but its flavor will improve with the longer, slower rise offered by refrigeration.
3) To make the sausage, combine all the ingredients. Refrigerate overnight, for best flavor.
4) To make the sauce: sauté the coarsely grated onion in the butter till it's beginning to color.
5) Add the crushed garlic, and sauté for about 30 seconds.
6) Stir in the tomatoes, oregano, sugar, and salt, and simmer gently for up to 1 hour, to concentrate the flavors.
7) To assemble the pizza: Divide the dough into two pieces. One should be about three-quarters of the dough; the other, one-quarter. If you have a scale, one piece should weigh about 36 ounces; the other, about 13 ounces.
8) Divide each piece of dough in half again. Butter the bottom and sides of two 9" x 2"-deep round cake pans, then drizzle olive oil in the bottom of each.
9) Stretch, then roll one of the larger pieces of dough into a round large enough (about 15" to 16") to line the bottom and sides of one pan, with some overhang. Do the same with one of the smaller pieces of dough, rolling it to about 9" to 10". Cover the pieces of dough, and go away for 15 minutes. This will relax the dough's gluten, allowing you to handle it without it shrinking.
10) Once the first two pieces of dough are rolled and resting, repeat with the other two pieces, covering them till you're ready to use them.
11) After 15 minutes, place one of the larger pieces of dough in the pan, pressing it gently into the corners; you'll have some overhang.
12) Spread half the uncooked sausage over the crust, gently patting it flat.
13) Top with half the sliced mozzarella. Spread with half the spinach, or any additional filling of your choice.
14) Place the smaller piece of dough atop the filling. Fold over the overhanging edges of dough, and squeeze/crimp to seal. Poke holes all over the top crust, to allow steam to escape.
14) Repeat with the remaining dough and ingredients, making another 9" round pizza.
15) Preheat the oven to 425°F (with your pizza stone on a lower shelf, if you're using a stone). While the oven preheats, allow the pizza(s) to rest/rise, covered, for about 30 minutes.
16) Just before baking, top the pizzas with the sauce, and sprinkle with the grated cheese.
17) Bake the pizzas till the crust is golden brown and the filling is bubbly, 20 to 25 minutes.
18) Remove the pizzas from the oven, and allow them to set for about 15 minutes. Loosen their edges, and gently turn them out of the pan onto a rack to cool. To do this, place a round cooling rack atop one pan, and turn the whole thing over. Lift off the pan, place a rack on the bottom of the pizza, and turn the whole thing over again, so the pizza is now right side up.
18) Serve warm, with a fork. Use a pair of scissors or baker's bench knife to cut wedges.
Yield: two 9" deep-dish pizzas, about 16 servings.