Heat oven to 475°F. Sprinkle baking sheet with cornmeal or line with parchment paper. On lightly floured surface, shape pizza dough into large oval. Transfer to prepared sheet and sprinkle with all but ½ cup fontina. In large bowl, toss mushrooms with balsamic vinegar. Add Brussels sprouts (whole leaves and slices) and onion, drizzle with oil and season with ½ tsp each salt and pepper. Toss to combine and scatter over dough. Sprinkle with remaining fontina, then crumble goat cheese over top and sprinkle with thyme. Bake until crust is deep golden brown and vegetables are tender, 10 to 12 min.