FWIW, on Mt. Sinai's stage list, pizza would be stage 2 after baked muffin, and mac and cheese would be stage 3 after pizza is passed. They've figured out the stages by the amount of milk protein in a serving of food. Since your allergist said it was ok, I would do as Carefulmom suggested, and buy a frozen pizza and overcook it a little. Again, FWIW, Mt. Sinai recommended Amy's brand. That's the one my ds has been eating since he began this journey. They do have a warning made in a facility with tree nuts, but if I recall correctly, it's only pine nuts, and they have very strict cleaning procedures.
If you still feel uncomfortable, would your allergist let you bring the pizza into the office for a 1x challenge?