Special equipment: a pizza stone and pizza peel
Place a pizza stone in the oven; preheat to 450 degrees F.
For the meat pizza: Heat a large skillet over medium-high heat. Add the sausage, breaking it up with a wooden spoon, and cook until cooked through, about 10 minutes. With a slotted spoon, remove the sausage from the pan, leaving any liquid behind. Set aside.
For the veggie pizza: Heat the olive oil in a large skillet over medium heat. Add the onions, peppers and mushrooms. Saute until softened, about 8 minutes. Remove from heat.
Sprinkle a pizza peel with cornmeal. Make one pizza at a time: Stretch and roll out each dough ball to a circle just smaller than the peel, then transfer the dough to the peel. Top each pizza with half of the marinara sauce, spreading it evenly over the dough with the back of a spoon; then sprinkle with half of the mozzarella cheese. Top as desired with meat or veggies, or just cheese. Slide the pizza onto the pizza stone and bake until the crust is golden and the cheese is bubbling, about 20 minutes.