Preheat oven to 210 degrees C. Sprinkle a large baking tray with polenta and place pizza base on top. Sprinkle parmesan over the top. Bake until parmesan starts to brown and pizza begins to puff, about 15 minutes.
Whisk vinegar, mustard, brown sugar and salt in a large bowl. Add tomato and onion and toss to combine. Set aside.
Top pizza with mozzarella and return to oven. Bake until cheese has melted and pizza base is crisp, about 5 minutes.
Add salad greens to tomato mixture; toss to coat. Scatter salad over hot pizza and serve.