Make the pizza: Place an inverted baking sheet or pizza stone on the lowest oven rack; preheat to 475 degrees F. Stretch the pizza dough into an 11-by-15-inch rectangle on a floured piece of parchment paper; trim any excess paper around the dough. Transfer the dough (on the parchment) to a pizza peel or another inverted baking sheet and slide the dough and parchment onto the hot baking sheet. Bake until browned in spots, 10 to 12 minutes.
Meanwhile, heat the olive oil in a medium skillet over medium-high heat. Add the sausage and cook, breaking it up into pieces with a wooden spoon, until no longer pink, about 4 minutes.
Remove the crust from the oven. Spread the ricotta on top, leaving a 1-inch border. Sprinkle with the mozzarella, provolone and parmesan; top with the sausage and garlic. Return the pizza to the oven and bake until the cheese is bubbling, 8 to 10 minutes. Sprinkle with the oregano.
Meanwhile, make the salad: Soak the red onion in ice water for 10 minutes, then drain. Toss the spinach, fennel, roasted red peppers, artichoke hearts and red onion in a large bowl with the vinegar and olive oil; season with salt and pepper.
Top the pizza with the salad and cut into pieces. Sprinkle with red pepper flakes.