Because the oven will not be a normal pizza baking kind of hot it can be a good idea to pre cook some vegetables before using them as toppings. Sauté or bake whole Portabello mushrooms and slice them up before using; sauté or bake zucchini’s, aubergines (eggplant), pumpkin, smoked tofu, asparagus, whatever you think will not get nice a soft as raw toppings under the given oven temperature and cooking time.