Preheat your oven to 400ºF (200ºC).
Line a baking sheet with parchment paper (I also sprinkle some fine cornmeal or semolina on the parchment paper to further prevent sticking, but that's up to you).
Prepare your pizza crust or take your store bought one out of the fridge. Let it rest for 5 minutes before shaping. You might want to roll it around in a little bit of olive oil first to make it easier to handle. For a personal pizza, I use a ball about the size of my fist. The rest of the dough I stored in the fridge in plastic wrap for the next day. My trick is to make a circular ball then stretch it over the rim of a medium-sized bowl. Once it's stretched over the bowl you can pull out the sides a bit more, then pick it up off the bowl and transfer it to the parchment paper. You want it to be thin, less than a 1/4 an inch. You might think then it won't hold the toppings, but once it cooks it will rise a bit, get crispy and be fine.
In a small bowl or cup combine the olive oil, rosemary and garlic, then brush over the whole crust. Slice the potatoes about 1/4 inch (1cm) thick lengthwise and spread out over the pizza, top with the onions and optional mushrooms, a bit of salt and then the vegan cheese.
Bake about 10-15 minutes until the crust is cooked and the cheese is melty.