Mushroom-Brussels Sprout Pizza

Flour, for surface

1 lb. refrigerated (or thawed from frozen) pizza dough/ cauliflower crust

3 oz. fontina cheese, coarsely grated, divided

4 oz. shiitake mushrooms, stems discarded, torn

1 1/2 tbsp. balsamic vinegar

4 large Brussels sprouts, trimmed, loose leaves separated, remaining thinly sliced

1 small red onion, sliced

4 large Brussels sprouts, trimmed, loose leaves separated, remaining thinly sliced

1 small red onion, sliced