Place a baking stone or overturned baking sheet on the lower-middle rack of the oven and preheat to 450°F (230°C).
Heat the oil in a large, heavy-bottomed saucepan over medium-high heat. Add the onions and cook, stirring occasionally, until deep golden brown, about 20 minutes. Add the vinegar and stir until evaporated. Season to taste with salt. Transfer the onions to a bowl to cool, then add the spinach leaves to the saucepan; cook, stirring frequently, until just wilted. Set aside.
Stretch the pizza dough to your desired shape and size. Remove the baking stone or sheet from the oven, dust with cornmeal or flour, and position the dough on top. Brush the dough with olive oil; top with the mozzarella and Parmigiano cheeses, then the onions and spinach, and dot with the goat cheese. Bake about 15 minutes, until the crust is golden and the cheese is bubbling in the centre.
Notes
Low and slow onions: 20 minutes to deep golden builds sweet, jammy flavour; the vinegar adds balance.Wilt the spinach: Cook and set it aside so excess moisture doesn't make the pizza soggy.Preheat the stone: A hot baking stone or overturned sheet gives a crisp bottom crust.Dot the goat cheese: Add it in little pockets so you get creamy tang throughout.