Heat 4 tablespoons of the olive oil in a medium saucepan over medium heat. Add the onions and cook until softened, stirring occasionally, 8 to 10 minutes. Add the garlic and cook until softened, stirring occasionally, 3 to 4 minutes. Add the sugar, oregano, tomatoes and red pepper flakes. Stir to combine and season liberally with salt and pepper. Bring to a boil over medium-high heat then reduce to a simmer. Simmer, covered, for 30 minutes. Discard the oregano and season additionally with salt and pepper if needed. Cover with a lid and reserve.
Preheat the oven to 400 degrees F with a rack on the bottom shelf.
Heat a skillet over medium-high heat with 2 tablespoons of the olive oil. Add the sausages and cook until lightly browned, using the back of a wooden spoon to break up the meat, 4 to 6 minutes. Remove the cooked sausage to a paper-towel-lined plate to absorb any excess oil. Reserve.
Grease the inside of a 9-inch nonstick cake pan with the butter. On a lightly floured flat surface, stretch or roll 1 1/2 pounds of the pizza dough into a 1/4-inch-thick round. The round should be slightly larger than the pan. Place the round into the pan and use your fingers to lightly press the dough into the edges of the pan while also stretching the dough up the sides of the pan so that there is a slight overhang. Evenly lay the cooked sausage onto the dough, then top with the mozzarella and Cheddar. Stretch or roll the remaining pizza dough into a thin round that is just slightly larger than the pan. Cover the top of the pan with the pizza dough. Crimp and cut the edges like you would a pie. Use a sharp paring knife to cut 6 to 8 slits on top of the pizza. This is very important or your pizza might explode! Cover the top of the pizza with 1 cup of marinara sauce, leaving a 1/2-inch border. Sprinkle with the Italian seasoning and brush the remaining 1 1/2 teaspoons olive oil on the edges of the crust.
Place the pizza on the bottom rack of the oven and cook until the edge of the crust is golden brown and the cheese is melted, 50 to 55 minutes. A good way to check if the cheese is melted is to put a cake tester into the middle of the pizza and then put the cake tester on your wrist. If the cake tester is hot, the cheese is also hot (and melted!). Remove from the oven and rest for 15 minutes. To serve, rewarm the marinara over low heat, adding 1/2 cup water to thin the sauce slightly. Serve with the hot stuffed pizza.