If pizza dough has been refrigerated, remove from refrigerator; let stand 20 minutes.
Preheat oven to 425°F. In a medium skillet cook sausage until no pink remains; drain and set aside. Trim ends and yellow leaves from Brussels sprouts; slice crosswise 1/4-inch thick, reserving any leaves that fall off.
On a floured surface roll out pizza dough to a 12-inch circle or 14x8-inch rectangle. Transfer to a lightly-oiled pizza pan.
Spread tomato paste over dough to within 1/2-inch of the edge. Sprinkle with salt and rosemary.
Top with 1/2 cup of mozzarella, the cooked sausage, Brussels sprouts, onion, and remaining 1/2 cup mozzarella. Bake until crust is golden and cheese is well melted, about 15 minutes.